Leonard Hofstadter

Dr. Leonard Leakey Hofstadter , Ph.D., is an experimental physicist at the California Institute of Technology who formerly roomed with Sheldon Cooper in Apartment 4A and now he and his wife Penny live in that apartment and they also formerly lived in Penny's apartment Apartment 4B. Along with Sheldon, he is the main male protagonist. Originally from New Jersey, Leonard attended Princeton University and was twenty-four years old when he received his doctorate with a Dissertation of the Year award for. Having lived with Sheldon for ten years as of Season 6, he and Leonard are best friends; however, Leonard can become very agitated by many of his and often feels obligated to explain Sheldon's behavior and serve as a  between him and other people in situations – he has been described as the  of the series. While Leonard, along with his geek friends and fellow scientists, is somewhat socially inept, Leonard would like to be better accepted outside of his social group. Leonard is better-adjusted than his companions, being more willing to try different things. He appears to be best suited as a leader of the group, with Sheldon commenting how Leonard is the "Kirk" of their landing party. The romance between him and neighbor Penny, with their on-and-off relationship has been the major force for drama until Shamy started.

Early Life (Before the series-2007)
Leonard was born on May 17, 1980 in New Jersey to Alfred and Beverly Hofstadter (Same month that erupted and "" came out.). Not much is known about his early life. He has two siblings: a younger brother named Michael and an elder sister that we don't know the name of ("The Maternal Capacitance", S2E15).

At age five, Leonard dressed up as, using his mother's make-up and made-up muscles (balloon muscles) with a bra of his mother's to stop them from sagging. He had dressed up as The Hulk because he was always mad at his mom and he wanted to smash his way out of the house and at age six, he walked in on his mother and father naked. She was swatting his father's bottom with Leonard's brand new ping-pong paddle and Leonard thought it was a dream until years later ("The Raiders Minimization", S7E4). From that same age, he had been dreaming about spending with a girl ("The Large Hadron Collision" (S3E15)).

At age eight his mother humiliated him by making him give back a ribbon at a science fair. He said it was his first science fair and that he had to give the ribbon back because he had done "Do Lima Beans React Better to Classical Music?". His brother had previously done "Do Lima Beans Grow Worse to Rock N' Roll?" ("The Pants Alternative", S3E18). At age ten, he built a hugging machine to compensate for lack of affection which his father used to borrow ("The Maternal Capacitance", S2E15). His father was an who spent more time with a 2000-year-old skeleton of an Etruscan boy than with Leonard, who then said "I hated that kid" when he recalled this fact in "The Fish Guts Displacement", (S6E10).

He did not celebrate his birthday or Christmas as a child ("The Peanut Reaction", S1E16) due to his mother having the family study the holidays for their "anthropological and sociological implications on society" and only celebrating achievements in the family ("The Peanut Reaction" (S1E16) & "The Maternal Congruence", S3E11). The family wrote research papers for and they were returned graded with his stocking. ("The Santa Simulation", S6E11) As Leonard was a geek, he was bullied badly as a youth and he stopped growing in the eighth grade. Throughout his childhood, his mother constantly conducted tests on him. One Easter his mother sent him out on an Easter egg hunt without putting out any eggs to see how long he would search for them (It took until June to figure the scheme out) ("The Raiders Minimization", S7E4). Before he went to Princeton, his parents put him in advanced placement classes, and sent him to learn, "getting me beaten up enough", as he described himself ("The Hamburger Postulate", S1E5). Johnny Galecki does play the cello. Leonard had his first kiss at age 19 to a girl named Geraldine Coco (not Cuoco).

After graduating from high school, Leonard attended Princeton University and got his doctorate at age 24. In 2003, he met Sheldon Cooper, with whom he began sharing an apartment, but not before meeting Howard Wolowitz and Rajesh Koothrappali. The 4 guys became a group and hung out often. On June 30, 2004, Leonard and Howard made a Girlfriend Pact. The pact was that when one of the two got a girlfriend, they would have that girlfriend set the single guy up with a female friend of her own ("The Creepy Candy Coating Corollary" (S3E5)).

Season 1
Leonard, Sheldon, Howard and Raj meet Penny and Leonard instantly falls in love with her ("Pilot" (S1E1)).

Season 2
His first date with Penny went well, but they broke up shortly afterwards because Penny thinks that he'll only date a girl with a college degree. He goes on to date Stephanie and Leslie, but both of those relationships don't last very long (Stephanie and Leonard broke up after only three episodes, while Leslie dumps him and then starts a friends-with-benefits relationship with Howard). Leonard finds out near the end of the season that Penny has feelings for him. Just before he leaves for his trip to the North Pole, she gives him a thick blanket, along with a hug. After he leaves, she tells herself, "I wish you weren't going."

Season 3
Leonard and Penny start dating, though at first they have some trouble trying to be more than friends, but they eventually start a stable relationship.

In "The Creepy Candy Coating Corollary", while in bed, Leonard tells Penny about his Girlfriend Pact with Howard meaning that if one of them got a girlfriend, she would set the single one up with one of her friends. Later, Howard meets Bernadette who is not able to find anything in common with Howard until they both find out they have overbearing mothers.

While in bed one night after sex, Leonard admits to Penny for the first time that he loves her. Penny is troubled by the potential serious turn in the relationship because she has commitment issues and can not respond in kind, and she breaks up with Leonard. They do work things out to remain friends.

Season 4
Leonard begins a relationship with Raj's sister Priya. Penny is devastated. She admits that Leonard is a great guy and she should have never broken up with him.

Season 5
Leonard ends his relationship with Priya after learning that she slept with her ex-boyfriend. In the 100th episode, he reconnects with Penny and they resume their relationship, establishing a "beta-test" to ensure that they don't face the same problems as the last time they were together. At the end of the season Leonard asks Penny to marry him for the first time.

Season 6
Penny almost breaks up with Leonard because, to her, the relationship seemed boring; however, she visits him at work and is re-impressed with how smart he is and their relationship seems to take off from there. They become more committed to, and dependent upon each other during the rest of this season. Penny eventually admits to Leonard that she loves him, and soon faces serious issues when Sheldon's new assistant gains a crush on Leonard, but the issues diminish when Leonard assures Penny that he's not turning his back on her. Leonard leaves for a North Sea expedition in the season finale, with the two of them now comfortable enough in their relationship to say they love each other.

Season 7
Upon his return from the trip, Leonard and Penny's relationship starts to get very serious. They do plenty of gestures for each other, and at the end of the season they become engaged to be married.

Season 8
Leonard and Sheldon come up with a new scientific theory that gives them recognition, while at the same time, Penny and Leonard's relationship continues to improve, including a minor situation regarding finances and living arrangements that won't upset Sheldon. In the last episode of the season, thanks to some prodding by Sheldon to set a date for their wedding, Leonard and Penny take off to Las Vegas to get married.

Season 9
Leonard and Penny arrive at a Las Vegas wedding chapel. They later hear about the Shamy breakup and get married with their friends watching over webcam, but only Howard and Stuart catch their "Husband and Wife" pronunciation part. They then have a terrible fight about Leonard's accidental indiscretion and return to their own apartments (The Matrimonial Momentum). Leonard meets up with Mandy to talk with her and later, he and Penny reconcile and Sheldon gives them a wedding gift: Tickets to San Francisco (The Separation Oscillation). In "The Positive Negative Reaction", they discuss having children one day. Because of the surprise elopement not sitting well with everyone, Penny and Leonard decide on having another ceremony to renew their vows, with their friends and family attending.

Season 10
Leonard and Penny have their re-wedding ceremony at a park which is officiated by Bernadette (The Conjugal Conjecture).

In "The Fetal Kick Catalyst", Penny is signing autographs for fans of the first and second Serial Ape-ist movies where the fans think that she is a terrible actress and can't believe that the nerd Leonard is her husband. The fans sit around Leonard as he tells his story how he won the hot actress, which makes them think of him as awesome.

Leonard and Penny finally have their own apartment: Apartment 4A, with Sheldon having moved out to live with Amy in Apartment 4B, first as a living experiment (The Cohabitation Experimentation), but then permanently (The Veracity Elasticity).

Season 11
In "The Proposal Proposal", Leonard is happy about Sheldon and Amy's engagement. Bernadette suggests that Leonard and Penny should have a baby in order to try and calm Howard and Bernadette's worries about having another baby so soon, but Leonard and Penny declined.

In "The Retraction Reaction", Leonard is interviewed by Ira Flatow on, but screws up in the interview. Later, it seemed that he was right about what he said.

In "The Matrimonial Metric", Sheldon picks Leonard to be his best man for his wedding. He and Penny are Shamy's wedding party.

Season 12
In "The Decision Reverberation", Leonard starts to be more confident in himself and talks President Siebert into making him a co-lead on a new experimental physics project.

In "The Maternal Conclusion", Leonard finally makes amends with his mother after all these years.

In the last episode, "The Stockholm Syndrome", Penny learns that she is pregnant. One smart and beautiful child in the oven.

Career
Leonard is a senior experimental physicist at the California Institute of Technology and has been published in several peer-reviewed journals. His honors include the Newcomb Medal. When he started at the university seven years ago as of the third season, he conducted classified government research on military rocket fuel capable of generating over 8,000 kN of thrust. His work often includes the use of high-powered lasers, and his research topics have varied from Bose-Einstein condensates and fundamental tests of quantum mechanics to cosmic radiation and dark matter. According to Sheldon, most of his work is extremely derivative.

With Sheldon, he co-authored a paper entitled "Paradoxical Moment-of-Inertia Changes Due to Putative Super-Solids", to be presented at an Institute for Experimental Physics topical conference on Bose-Einstein condensates. The findings contained therein show that at temperatures approaching absolute zero, the solid becomes a super-solid, a previously unknown state of matter, and the moment of inertia changes.

To study the soft component of cosmic radiation at sea-level, Leonard designed an experiment using a helium-neon laser. When the university received a new free-electron laser, he required access in order to perform his X-ray diffraction experiment. Subsequently, he procured a big government grant to see if high-powered lasers can be used to knock out incoming ballistic missiles, though he considered the concept improbable. After progress with his ongoing research regarding high-energy lasers, he worked on a front-projected holographic display combined with laser-based finger tracking.

His investigations into dark matter were rendered useless by Dr. David Underhill's observation of high-energy positrons, the first conclusive evidence for the existence of galactic dark matter. Yet, together, they examined the radiation levels of photo-multiplier tubes for a new dark matter detector. He later attempted to replicate the dark matter signal found in sodium iodide crystals by the. Likewise, he has grown isotopically pure crystals for neutrino detection and built, with the aid of Howard Wolowitz, a multi-wire proportional counter for detecting cosmic particles.

Leonard has carried out a series of anti-proton decay experiments with negative results. His experiment consisted of twenty thousand data runs and no statistically significant results.

Dr. Hofstadter's tests of the Aharonov-Bohm quantum interference effect reached an interesting point when examining the phase shift due to an electric potential, wherein he set up the voltages using tunnel junctions. Though, given the parameters of his experiment, the transport of electrons through the aperture of the nano-fabricated metal rings was qualitatively no different than an experiment already conducted in the per Sheldon and Penny. Their observed phase shift in the diffusing electrons inside the metal ring already conclusively demonstrated the electric analogue of the Aharonov-Bohm quantum interference effect. On the other hand, collaboration with Dr. Arthur Jeffries on a paper about nano-vacuum tubes was promising.

Since Stephen Hawking knew Dr. Hofstadter's research, Leonard was invited to join the professor's team, which sent an expedition to the to test hydrodynamic simulations of. Leonard spent a period of four months looking into the possibility that one could find the equivalent of in a large body of water, as the similarities of the equations of general relativity and hydrodynamics suggest.

Based upon a theory from Leonard, Sheldon wrote a paper detailing a theory that space-time is like a super-fluid. The paper was well received by the scientific community that included Professor Stephen Hawking. The theory is discussed in article in "" and "", the former didn't mention Leonard. Also, Leonard and Sheldon were invited to the to lecture on their work, but missed it after they were detained for an incident at '  headquarters.

In "The Retraction Reaction", Leonard is interviewed on NPR and describes all these theories, but admits that physics hasn't found anything new recently. He gets called into Janine Davis' office and is told to issue a public retraction non his comments. After all the guys gets depressed and wonder what they are doing professionally after it seemed that Leonard was right, they go to visit the grave of Richard Feynman which rekindles their love of science and working at it for the fun of it.

In "The Novelization Correlation" Leonard is writing a book about a physicist solving a murder at the CERN collider in Switzerland.

In "The Grant Allocation Derivation", he has to distribute some excess grant money which he takes very seriously. After turning down many requests looking very decisive, he can't decide between the last three so he buys himself a new laser.

In "The Decision Reverberation", Penny again tells Leonard that he should stand up for himself more. Leonard decides that he wants to be the principle investigator on a plasma physics study so he prepares a proposal and plans to threaten to quit if President Siebert will not accept his proposal. Penny is worried because she doesn't want to move. Even though President Siebert turned him down, Leonard does get called back and offered a different job as co-lead on a photo entanglement project. Leonard is happy and Penny is very proud of him.

Characterization
Although Leonard feels at home with his geek colleagues, he is reluctant to let others know about his recreational activities (such as Klingon Boggle), as he does not like to be seen as a geek and wants to be more social. Sweet, thoughtful, kind, intelligent, mature, sardonic and socially awkward, but well-meaning, Leonard also seems to be the most well-adjusted and sensible one of The Social Group, and is quick to try new things apart from his geek friends.

Despite his reasonable nature, he can sometimes display selfishness, normally putting his needs before his friends' well-being. During the group's expedition to he tricks Sheldon into thinking that he had definitively proven slow moving magnetic monopoles when in reality it was simply static from him switching the electric can opener on and off as he they wanted to kill Sheldon. When they return from the North Pole he is totally humiliated among his scientific peers. Sheldon fled to Texas to live with his mother, Leonard, despite the trick being his idea, wanted Raj and Howard to go to Texas without him to convince Sheldon to come back, just so that he could have first sex with Penny ("The Electric Can Opener Fluctuation"). Another example is when he refuses for Zack Johnson to be Superman for the comic book store's costume party because Zack has actual muscles ("The Justice League Recombination"). Another example of Leonard's selfishness was when he was sleeping with Raj's sister Priya, and didn't seem to care how Raj felt about the situation ("The Cohabitation Formulation").

A lifetime of being denied and rejected of praise and acclaim that he unavoidably deserves (Because his mother had absolutely no regard or admiration for him and often ruined his infrequent moments of happiness as a child, and Sheldon relentlessly unknowingly demoralizes him and his accomplishments out of egomania and sheer apathy) has led to Leonard yearning for recognition for his intelligence, compassion and determination. He becomes extremely bitter when he discovers that a major science paper that he and Sheldon cooperatively wrote was published with only Sheldon's name (which was obviously Sheldon's fault due to Sheldon being an arrogant credit hog). He also admits to Sheldon that he is an egotist who needs his big ego to be constantly validated from people who are much less smarter than he is while presenting their paper to a class of graduate students.

As a result of how he was raised (Unloved and underappreciated, due to the lack of love from his parents, his mother's views of raising children as obviously opposed to his father's), he was not as intelligent as his siblings and his mother having been unloving, unsupportive and neglectful altogether and treating Leonard as a personal science experiment instead. Leonard was seen as not any happier than Sheldon (who grew up loved) and frequently insecure and sometimes unhappy. But that soon changed during his second relationship with Penny. She made him more confident during their relationship (Since Penny always gave him lots of love and affection and Leonard was only in love with her the day they first met) and by the time they were married, Leonard was much happier than he had ever been in his life, having married the love of his life, as said by Amy and Bernadette in "The Separation Oscillation" (even his mother thought that perhaps she was wrong that they shouldn't be together due to Penny's education status, but when she saw how happy Leonard had been since their marriage, which she had never seen before, she thought they were a good couple together).

Among the main characters, he generally has the least amount of difficulty interacting with "ordinary" individuals, as compared to Sheldon (who is unable to grasp social norms), Raj (who is unable to talk to women outside his family unless under the influence of alcohol until the end of season 6), and even Howard (who is often seen as "creepy" when flirting with women) since Leonard is the only one of them interested and quite adept in building interpersonal and social relationships with other people. Compared to his friends, he is also relatively the most successful with women.

Between Sheldon and him, Leonard is the only one who feels that he "should do something" when Penny begins to recount her recent breakup. Leonard usually blurts out the wrong things in front of women when he is nervous, such as when he's trying to invite Penny to lunch and mentions that curry is a natural laxative, or, as in "The Plimpton Stimulation" where he can't decide what to say he will often combine thoughts. i.e. "Hi-lo," instead of "Hi," or "Hello."

Despite his attempts to reject his geek status, Leonard owns, among other things, Star Wars hygiene products, an expensive Superman comic book collection, a "Battlestar Galactica" Colonial warrior's flight suit, and a that he calls a "glow stick". Early in the series, Leonard tried to get rid of his collectibles to show maturity and impress Penny; however, he changed his mind when he saw Penny was already dating another man.

Leonard is indicated to be proficient in literature, history, and music as well as the sciences - he is able to solve a woman's crossword puzzle in a matter of seconds (the answers were mostly literary in nature) and he can play the cello.

Having lived with Sheldon for many years, they are best friends, and he is obligated to explain Sheldon's irrational behavior and serve as a mediator between him and other people in many situations; however, Leonard often gets very irritated by much of his behavior. For instance, when Penny and Sheldon engaged in a fierce dispute, Leonard gave Penny the phone number of Sheldon's mother so she would call her son and scold Sheldon for his behavior, thus ending the fight, as shown in the episode "The Panty Piñata Polarization".

Leonard is also upset that Sheldon dislikes his favorite show, "", which Sheldon refuses to allow to be watched in his apartment. Like everyone, he dislikes his friend's habit of rattling off different trivial facts. Leonard's IQ of 173 is not as high as Sheldon's and the latter has scoffed at Leonard's whiteboard theories as common enough to be "scribbled on the wall of every men's room at MIT."

Leonard seems to get along well with Howard. Leonard and Howard even have a Girlfriend Pact, which Leonard invoked in "The Desperation Emanation" so that Howard would help him get a girlfriend. In "The Peanut Reaction", Howard tried to make Leonard's birthday a surprise, going as far as getting himself hospitalized, proving to Leonard how much Howard cares for him though Penny also proposed him the phone number of one of her slutty friends. Leonard also seems to get along well with Raj. Although Leonard does get irritated by much of Raj's impulsive and eccentric behavior, as in "The Griffin Equivalency" where Raj became pompous and self-absorbed after being chosen to be in People magazine.

Leonard has a lot of conditions. He is lactose intolerant, asthmatic, bespectacled due to myopia, prone to carsickness and migraines, gets a bloody nose after jogging, suffers from sleep apnea, allergies and a lot of ear infections, and possesses a genetic predisposition to heart disease. Leonard's primary health concern is his lactose intolerance. He avoids eating even small amounts of dairy products (including cheese, yogurt, and butter), as well as melon and desserts at The Cheesecake Factory, for fear of his intestines blowing up like a balloon animal and resulting in the emission of a noxious gas that Sheldon maintains can be weaponized. This digestive trait is often the subject of teasing. According to the pilot episode, he cannot process corn either.

Leonard is self-conscious about his childhood bed-wetting, particularly given his middle name, Leakey (he was named for noted British archaeologist and naturalist ). He similarly despises that the word "nerd" is in his first name.



Leonard is somewhat neurotic, insecurely tilting his head as he speaks nervously, as well as avoiding eye contact and squinting. When under great stress, his voice is elevated to a high-pitched whine, when he lies or gets sweaty. At times, Leonard's demeanor seems very low energy and somewhat irritable. Clumsy, he often bangs his head, which induces vomiting. Leonard is incredibly sensitive about his height (he stopped growing in eighth grade), although he enjoys pointing out that Howard is shorter.

Leonard is usually seen wearing his characteristic black-framed glasses, neutral-colored jackets or vests over a hoodie, low-cut black Converse sneakers, brown trousers, and physics-themed t-shirts. Along with questionable fashion choices, including mismatched color schemes, he sometimes applies an absurd amount of hair gel; however, his girlfriends often buy him new clothes, and he has put aside his corduroy suit for formal occasions to don a fancy silk shirt and vest or blazer, much to Penny's approval. His formal suits style improves with time. He always wears his hoodies because Sheldon has the thermostat set so low.

Over the course of the series, despite retaining his geeky habits, Leonard's confidence has grown and he has become more socially adept throughout the series. Leonard is also somewhat charismatic and a good leader in certain situations, which Sheldon shockingly recognizes.

Physical Appearance
Leonard is a short and a little thick man, originally in his late twenties, but he's in his late thirties at the end of the series. He is 5' 5" (1.65 m) with brunette hair and brown eyes. Penny thinks that his eyebrows are weird. His wardrobe includes black thick-framed glasses, a large variety of colored and patterned sweat shirts with hoods worn under a neutral colored jacket, brown trousers, and low-cut black Converse All Stars sneakers. Like Sheldon, Leonard often wears physics-themed t-shirts (under his other layers), but he has a much wider selection; many of his t-shirt themes deal with social issues such as renewable energy (wind farms) and recycling, as well as animal issues (rescue programs and themes around endangered species). Leonard's wardrobe has its own section on the Wiki. Sheldon once said, male members of Leonard's family get bald very early, yet neither Leonard or his father are bald nor have lost much hair.

Family
All of the members of Leonard Hofstadter's family are accomplished scientists, apart from his younger brother, Michael, who is a successful law professor. Leonard is actually the least successful of his family in their respective fields.

His mother, Beverly, is a psychiatrist and neuroscientist who has a personality identical to Sheldon's, including strict speech patterns, lack of social conventions, and attention to details. She is mainly responsible for the difficult childhood Leonard had. Due to his mother's views, Leonard never celebrated his birthday during his childhood, and when she visited him, she didn't see the point of viewing his unoriginal research since it had already been done.

She never gave him any sort of approval growing up and one time humiliated him when she forced him to give back an award at the school fair because it was too similar to his brother's earlier project resulting in his recurrent bed-wetting. In "The Pants Alternative," Leonard suggested the reason that his brother is 8" taller than him is due to his mother's lack of nurturing him as a child. In "The Engagement Reaction," Penny similarly suggests that Leonard's lack of nurturing from his mother "makes him desperate to please women." Leonard had mentioned to Penny that when he was young he built a "hugging machine" with two self-constructed remote-controlled arms in order to compensate for the lack of affection from his mother; the saddest thing was that his father used to borrow it too. Beverly also mentioned to Howard that Leonard's brother (Michael) and sister are more successful in their respective fields than he is, but she was not proud of them as they weren't "her" accomplishments. When he was six, Leonard had walked in on his naked parents while his mother was spanking his father's bottom with his new ping-pong paddle. Leonard had thought that that incident had been a nightmare of his youth. When Leonard told her about his engagement to Penny, she was willing to accept it because Sheldon was fond of Penny. In return, Leonard has a huge desire to harm his mother or have bad things happen to her, blames all the horrible stories and issues of his childhood on her, considers all her books to be garbage and as a result of his upbringing, Leonard is desperate to please women and has sometimes been quite insecure until Penny made him more confident, as said by Amy and Bernadette in "The Separation Oscillation". Beverly attends Sheldon's birthday party and gets hit on by Barry Kripke to Leonard's discomfort. Later, she visits and Penny tries to bond with her, resulting in a vow renewal ceremony since Beverly was disappointed about not receiving an invitation to their Las Vegas wedding. Leonard infuriates her by inviting his father. In "The Conjugal Conjecture", Leonard's father says that even though he and Beverly don't bring the best out in each other, something wonderful did come out of their relationship: Leonard. Beverly agrees. In the "The Maternal Conclusion", Leonard realizes that his mother will never change and he forgives her. She says that helps and they have a truly motherly embrace.

Leonard's brother, Michael, is a Harvard University law professor and is engaged and later married to the youngest appellate court judge in New Jersey history. Leonard also says that his brother is eight inches taller than him. Later in Season 8, it is revealed that he had recently argued a case before the Supreme Court. He has not yet appeared in the program.

Leonard's sister, whose name is unknown is a cutting-edge medical researcher working with adolescent gibbons to cure diabetes. She has even gone as far as successfully growing a human pancreas in one. Leonard also says that she is 38 years old and married in Season 4. She has not yet appeared in the program.

Leonard's father, Alfred, is briefly mentioned as an anthropologist. Sheldon pointed out that Leonard's middle name, "Leakey", comes from famed archaeologist Louis Leakey with whom Leonard's father had worked, which Leonard finds humiliating. It is implied due to Beverly's nature that Leonard prefers his father over her. In Season 3, his father had an affair, which resulted in their divorce. Although his relationship with his father is unknown, it can be presumed that Leonard prefers his father more. In "The Convergence-Convergence" Alfred appears and he and Leonard appear to get on really well, is happy for the couple and is very fond of Penny. After sniping with his ex-wife, he and Sheldon's mother really get cozy and disappear into their hotel together, an idea that Leonard and Sheldon do not like, although it's revealed in the next episode "The Conjugal Conjecture" that they didn't sleep together, but plan on still seeing each other in the future.



Leonard's now-deceased Uncle Floyd was briefly mentioned in "The Maternal Capacitance" as the only person from his childhood of whom he had fond memories. In "The Codpiece Topology," though, he mentions his Aunt Edna, a very hairy woman, is sweet and would tickle him when they hugged. The closest thing he ever had to a birthday party was when his grandfather died on his birthday, so he got to see his cousins and have his favorite German chocolate cake.

There is male pattern baldness in his family. According to Sheldon, when his uncles sit around the dinner table they look like a half carton of eggs. At Thanksgiving, his grandmother with Alzheimer’s had an episode where she took her clothes off in the apartment; Leonard describing the worst part as when she carved the turkey. This took place before the series started.

In "The Itchy Brain Simulation," Leonard showed Penny a sweater that an aunt had made for him when he went off to college. It is bright red, very itchy and had "Lenny" spelled out on the front. At the same time as the sweater, his brother got a car from her.

In "The Celebration Experimentation", Beverly mentions that Sheldon was at her 60th birthday get together along with Leonard's siblings and he knew nothing about it. Penny counters that since their wedding was in Vegas, she did not get an invitation. Beverly is thankful for that. She backtracks on the wedding invitation a the end of the season.

In "The Explosion Implosion", after Penny answered a phone call from Beverly for Leonard, the pair developed a close bond with Beverly dishing on her recent amorous activities. Amy was jealous of Penny's close relationship with Leonard's mother, while Leonard was more annoyed that his mother told Penny she was proud of her - something Beverly has never said about him. When Leonard confronted his mother, Beverly told him how great Penny was, that out of all of her children's spouses, Penny was the best one and admitted she was proud of Leonard - if only for marrying well. Beverly also noticed that Leonard was trying hard not to cry, so she let him hang up quickly since Leonard was already being driven to tears.

In "The Maternal Conclusion", Beverly visits Leonard and Penny asking nice and truly interested in Leonard's new physics project. Later she admits that she is doing research for a new book which makes Leonard blowup. Finally he forgives her which makes both of them feel much better and they end in a true loving embrace.

Family Members

 * Father: Alfred Hofstadter
 * Mother: Beverly Hofstadter
 * Brother: Michael Hofstadter
 * Sister: Unnamed
 * Uncle: Floyd
 * Aunts:  Edna, Nancy, Unnamed
 * Cousins: Unnamed
 * Grandfather: Unnamed
 * Grandmother: Unnamed
 * Wife: Penny Hofstadter (née ?)
 * Child: Unborn
 * Father-in-law: Wyatt
 * Mother-in-law: Susan
 * Brother-in-laws: Randall, Unnamed (via Penny's sister), Unnamed (via his own sister)
 * Sister-in-laws: Unnamed (via Penny), Unnamed (via Michael)
 * Nephews: Neil, Jeffrey, Scott, William, Richard (all via his sister)
 * Nephew: Unnamed (via Penny)
 * Unnamed nephews/nieces
 * Uncle-in-law: Unnamed
 * Cousin-in-law: Unnamed
 * Paternal Grandfather-in-law: Unnamed
 * Paternal Grandmother-in-law: Unnamed
 * Maternal Grandmother-in-law: Unnamed
 * Maternal Grandfather-in-law: Unnamed

Sheldon
Sheldon is the best friend of Leonard, as they live together and are accustomed to tolerating each other's quirks ever since Sheldon put out an ad for a roommate with a list of criteria to follow. When Leonard first met Sheldon he had a bunch of rules that Leonard had to follow. Despite Sheldon sometimes making remarks to the contrary, he appreciates Leonard and assures Leonard that his friend will not die alone. He even once told Leonard that he platonically loves him, when Leonard had to face his old high-school bully, Jimmy Speckerman. When Penny asks Leonard why he deals with Sheldon, he explains that Sheldon has actually saved his neck a few times, such as when he stopped him from being incinerated by his own improperly formulated rocket fuel and didn't tell on him to the landlord, the police or the. This incident also lead to the destruction of the elevator, as Sheldon had just enough time to put the explosive contents of the container into the elevator before it exploded. Leonard also, more than all of Sheldon's other friends understands him the most, as viewed in many scenes where when all hopes failed to get Sheldon to participate in something, Leonard will be the last resort to come up with something successful. When Penny and Sheldon had a spat due to Penny sitting in his spot and touching his food and later refused to apologize to him, which resulted her being banished from the apartment, Penny came up with her own set of revenge, including banishing Sheldon from the Cheesecake Factory and messing up with Sheldon's Laundry Night. When things got really ugly between the two, a fed up Leonard secretly gave Penny Sheldon's mother's number, telling her it is Sheldon's. The feud ended after that, with Sheldon, clueless to the fact that it was Leonard who helped Penny, told Penny that she played well. Another instance was when Penny tried her hands at opening her own business by selling Penny Blossoms. When they had to stay awake all night to help Penny meet her order, a sleepy Sheldon refused coffee and decided to head to bed instead, leaving the rest to finish up by themselves. A frustrated Penny looked on to Leonard to do something and Leonard did, by saying to Sheldon that without his insight and leadership this entire enterprise will surely fail, which prompted Sheldon to change his mind and drink coffee. In "The Septum Deviation", Sheldon objects to him have elective nasal surgery due to the risks of surgery indicating that he really cares for Leonard despite his condescension. Sheldon and Leonard have co-authored at least two scientific papers together.

In "The Proton Displacement" (S7E07), Leonard explains why he puts up with Sheldon and takes Sheldon as his friend:


 * "Look, I know he can be aggravating, but what you have to remember is that he is not doing it on purpose. It's just how he is. He is also loyal and trustworthy, and we have fun together...You know what, Sheldon is the smartest person that I have ever met. He is a little broken and he needs me. I guess I need him, too."

In "The Space Probe Disintegration", Leonard tells Sheldon that his attitude is why he and Penny are not living together. After a tearful period thinking about them him moving out, Sheldon lets him go one night a week to stay at Penny's. Sheldon still sleeps on her couch that night.

In "The Skywalker Incursion" the guys drive up to ' headquarters to see if they could get in. They can't so Sheldon rushes the gate, gets caught by being tased and almost gets the two of them arrested. Leonard is mad, but still calls it an adventure.

In "The 2003 Approximation", Leonard finally moves out to live with his wife Penny; however, in the end Penny agrees that they will spend at least half their night back in Leonard's old room.

In "The Convergence-Convergence", Sheldon's mother and Leonard's father find a connection, may have spent the night together and Leonard could end up Sheldon's step-brother. When Leonard toasts Sheldon on his birthday in "The Celebration Experimentation", he calls Sheldon his brother.

In "The Matrimonial Metric", Sheldon and Amy test their friends for positions in their wedding party. The gang gets pissed and no one wants to be in their wedding. Later Leonard tells him that his best man is his own choice and Sheldon choses him.

In "The Bow Tie Asymmetry", Leonard is very happy for his buddy Sheldon when he is about to get married. He is also happy that Sheldon was now legally Amy's problem, but Sheldon says that that is not going to stop him from being his problem. After the wedding ceremony is delayed, Leonard checks up on them and finds Sheldon and Amy inventing Super-Asymmetry which also gets Leonard fascinated and involved. Penny has to come and get the three of them to start the wedding.

In "The Citation Negation", Leonard helps Sheldon with the footnotes in his Super-Asymmetry paper and finds an old untranslated Russian paper that states that Super-Asymmetry is not worth pursuing.

Howard
Howard meets Leonard in a flashback in "The Staircase Implementation" and they are best friends. Howard told Penny that both he and Raj liked Leonard, not Sheldon. Leonard has helped him quite often, and Howard faced an allergic reaction partly to provide Leonard's first birthday party. Though, Howard finds Leonard's worries over Penny humorous sometimes, and they like to post embarrassing things about each other on YouTube and Twitter. The two of them made a pact that if either one of them ever got a girlfriend, the other one would set up the single guy with one of her girlfriends. When it comes to agreements and women, they frequently fight. Leonard does enjoy poking fun at Howard's height, although he is only a quarter of an inch taller ("and don't you forget it!"). In "The Conference Valuation", Leonard spends the weekend at Wolowitz's playing board games and conducting development tests on Howard's kids. Leonard has some deja vu because he sees that Sheldon treats the kids the same way Beverly treated him, testing him all the time.

Raj


Leonard and Raj meet at the University in a flashback in "The Staircase Implementation" and they are best friends. Made clear in "The Jerusalem Duality", both Raj and Howard like Leonard. Though, Raj takes delight in Leonard's problems with Penny and becomes somewhat adversarial with him when he dates his sister, Priya. Despite his tendency to tease Leonard about his girlfriend issues, he is supportive of his and Penny's relationship and frequently attempts to get him to break up with Priya (sometimes employing subtle manipulative techniques, such as feeding his paranoia about the two women comparing notes about his sex life). Raj feels jealous when Leonard and Howard use his ideas for fun and exclude him. Raj has said that Leonard only hangs out with him if he has no other options, and has missed him ("The Spaghetti Catalyst"). When Sheldon and Penny were indisposed, Leonard took the very appreciative Raj with him to  for a visit to the Large Hadron Collider after both Penny and Sheldon fall sick [as seen in "The Large Hadron Collision". They normally try to help each other. In "The Date Night Variable", Raj hangs out with Leonard and Penny on their date and gets thrown out when he tries to get Penny to tell Leonard that she loves him. In "The Cooper Extraction", Amy points out to the gang that most of them would not know each other if Sheldon had not been part of their lives. Since Leonard is not in Apartment 4A, Raj envisions them living together; however, Leonard is fat due to Raj's cooking and Leonard's loneliness.

Bernadette
Leonard and Bernadette have a warm relationship. They became friends shortly after Penny introduced her to Howard. She became interested in Leonard's experiments, but they are only best friends, as Howard and Bernadette are in a relationship (even though Howard felt threatened). Leonard is impressed how she can get Sheldon to do things such as go to bed, but then her mother ran an illegal daycare center and she was used to dealing with stubborn children ("The Einstein Approximation"). In a daydream of Raj in "The Thespian Catalyst", Leonard comforts Bernadette after he and Raj hear about Howard's accident when he was on his scooter and hit by a truck and she started crying. In "The Scavenger Vortex", Leonard and Bernadette are matched in their scavenger hunt and she keeps pushing him around because she is very competitive.

But despite problems, Bernadette and Leonard truly love and care for each other, and Bernadette even shows her love for Leonard in "The Conjugal Conjecture" where she officiates Leonard and Penny's re-wedding ceremony when she says "The love we have for them" with the 'we' referring to herself and the guests and the 'them' referring to Leonard and Penny; however, up to the Season 10 finale "The Long Distance Dissonance" there hasn't been any good moments of them being good to each other, either with their friends present or when they're alone together, showing that their cordial relationship seems to be going downhill.

In "The Proposal Proposal", Bernadette speaks to Leonard sweetly and smiles at him with nothing terrible happening afterwards, so this also shows that she truly loves and cares for him.

Amy
Amy and Leonard did not get on very well at first, but now they are best friends, with Leonard finding her judgmental and obnoxious like Sheldon and Amy seemed to always look down on him. Leonard slowly warmed to Amy, though they rarely hang out alone. Leonard found it odd when Amy stayed late at the apartment, even after Sheldon had left, oblivious to the fact Leonard would have preferred to be alone. Their relationship improved gradually. Finally, in "The Pulled Groin Extrapolation", Amy invited Leonard as her date to a wedding she was about to attend, explaining that Sheldon was a terrible guest at a previous wedding because there were no physicists his age there. They ended up enjoying each other at the wedding and Leonard commented that Amy really knows how to make a guy loosen up and have a good time. When Amy called Sheldon while drunk in "The Isolation Permutation", Leonard and Sheldon went to see her at the liquor store parking lot at Leonard's insistence. Leonard ended up being the person helping Amy up and walked with her as they made their way to send her back to her apartment. Amy giggled when she said that he had found "second base". In "The Flaming Spittoon Acquisition", Stuart asked Leonard to be the guy who checks in with Sheldon regarding his response to him asking Amy out. In "The Status Quo Combustion", Leonard lets Sheldon go on his train trip and Amy explodes, hitting him with a pillow for letting him go. When Leonard stayed with Amy and Bernadette at a bar after they had lied to Sheldon and Penny, Leonard wondered why he and Amy didn't hang out together more often. Amy said it was weird when they did.

The two of them share some similarities: both have PhD's, both wear glasses, both had difficult childhoods caused by their mothers who they don't like (Leonard's being unaffectionate and Amy's being strict), they have both been hugged by Sheldon and he would hate to lose them, both have connections with Harvard (Leonard's brother Michael is a law professor and Amy was a student) and they are very devoted to their main lovers (Leonard = Penny & Amy = Sheldon).

In "The Celebration Experimentation", Sheldon ran into the bathroom after being overwhelmed by the birthday party they were throwing him. Leonard and Amy argue over who knew him better to talk to him. Leonard has known him longer, but Amy and Sheldon have seen each other naked. Penny goes instead.

In "The Solo Oscillation", Sheldon throws Amy out of the apartment so he can have some quiet time to work. She hangs out in Leonard and Penny's apartment and starts to bond with Leonard. They were both spelling bee champions and they bore Penny with a night of them trying to stump each other with spelling words. The next night they start reenacting old high school science projects like creating hot ice and building a wave front out of Popsicle sticks.

Stuart
Leonard and Stuart are very friendly with each other and Leonard regularly visits him at the comic book store. Stuart has also invited Leonard over to his store for parties. Stuart went on two dates with Penny, which of course irritated Leonard to no end even though she and Leonard were not currently in a relationship. The first date was in "The Hofstadter Isotope". The second date in "The Classified Materials Turbulence", Stuart looked to Leonard for advice in dealing with Penny. Leonard tried to avoid giving him advice but finally broke down. The advice, however, turned out to be exactly how Leonard had been trying to woo Penny for the last two years - thinking that if it didn't work for him; it wouldn't for Stuart. After the date, Leonard felt really bad and finally caught up with Stuart to apologize when Stuart actually appreciated the advice and it worked great right up until they were making out and he said, "Oh Penny!" And instead of saying, "Oh Stuart!" she called him Leonard. While acting sorrowful for Stuart, Leonard was overjoyed that Penny still liked him.

Emily
Leonard and Emily were on good terms, but haven't been seen alone together.

Wyatt
Wyatt is Penny's father and Leonard's father-in-law. Wyatt is very fond of Leonard. He sees him as the first smart boyfriend Penny has dated, while all her other boyfriends Wyatt considered losers, while Leonard is seen by Wyatt as a friendly, caring person and a successful accomplished academic. They first bond over playing Wii fishing and he begs Leonard to get back with Penny.

Susan
Susan is Penny's mother and Leonard's mother-in-law. Like her husband, Susan is very fond of Leonard.

Mary Cooper
Based on her mature, kind, caring and loving nature, Mary Cooper, Sheldon's mother, appears to be Leonard's female equivalent. Leonard always likes Mary and looks up to her as he always finds his mother to be pathetic. She is the mother Leonard wished he had. In "The Convergence-Convergence", Mary says that she has always had a special place in her heart for Leonard, taking care of Sheldon for years. It is likely she could end up becoming his stepmother because in that same episode she appears to have hooked up with his father. However, nothing happened between Alfred and Mary, much to Leonard and Sheldon's relief. Though everyone is disturbed at the thought of them still planning to maintain a friendship.

Bert
In "The Geology Methodology", Bert asks Sheldon to collaborate with him on a dark matter project involving meteorites which Sheldon likes, but is embarrassed to tell others about. When Bert learns this he stops working with Sheldon and replaces him with Leonard.

In "The Meteorite Manifestation", Bert wants Raj's help cutting open a meteorite with a band saw. Leonard wants to do it with his laser, but Bert declines. Leonard gets angry that he is left out. He dreams that he steals the meteorite, cuts it open turns into to a flesh eating zombie.

George Cooper Jr.
Leonard and Sheldon visit Texas to ask his brother to come to "Sheldon and Amy's Wedding". Leonard helps them reconnect and George does come to the wedding. Leonard also found out that George did for Sheldon a lot of the same things that Leonard does like explaining his behavior and driving him around. George Jr. appeared again in "The Tam Turbulence", where he talks to Leonard, Howard and Raj via webcam about Sheldon's childhood friend Tam, whom they are curious about.

Romantic
Leonard is usually mildly shy around women, but unlike Sheldon, he actually cares about having a girlfriend. He is the most successful of the main four to get into relationships. Although, quite shy around women at first, not really the 'outgoing' type when it came to relationships, he soon got better with women over time. Also he believed that he had to do anything the woman wanted to stay in the relationship until Penny set him straight. He is actually his most confident when he is in a relationship, examples include:


 * Being very natural with Alice and indirectly asking her out when he was in a relationship with Priya. ("The Good Guy Fluctuation")
 * Acting very kind (and oblivious) to Alex, unknown to him that she was flirting with him, while he was in a relationship with Penny. ("The Higgs Boson Observation", "The 43 Peculiarity" and "The Egg Salad Equivalency")
 * As Howard recounts, Leonard's relationships record prior to Stephanie Barnett, excluding the woman with a PhD in French Literature, is approximated by the geometric progression

$$y = \frac{27days}{12^{n}}$$

where n = 0, 1, 2; twenty-seven days with Joyce Kim, two booty calls with Leslie Winkle (2.25 days), and a three hour dinner with Penny which would have been two and a half if they ordered the souffle when they sat down (4.5 hours). His relationship with Stephanie should have ended after twenty minutes (22.5 minutes for n = 3).

Joyce Kim
Joyce Kim was one of Leonard's relationships that was widely talked about in his social circle, but was never seen until a flashback in "The Staircase Implementation". A spy who attempted to get information on the US government project Leonard was working on (a type of Rocket Fuel). The relationship lasted only 27 days before Joyce defected back to North Korea

Penny
Leonard is instantly captivated by the presence of Penny during the "pilot episode", and from that moment he sets his mission to date her. By the end of the pilot episode, he has already announced to Sheldon "our babies will be smart and beautiful." Sheldon is quick to add, "not to mention imaginary" though by series end, Penny is pregnant.

His infatuation with Penny becomes the major force that drives the series during the first season. They did go out to dinner one night with Penny thinking that the other guys were going to join them and Leonard making excuses for them. She asks later at her door if this night was a date; however, Leonard denies it though he doesn't mean it. At the season one finale, Leonard finally managed to ask Penny out, and they had their first date after Penny has a bad breakup.

Subsequently, they broke up because Penny felt she wasn't as smart as Leonard would like and that she'd need a college degree to date Leonard. On another occasion, during the visit of Leonard's mother, Leonard and Penny came very close to having sex, but he ruined the moment by talking about their childhood issues with his mother and her father, respectively.

In the second season finale, when the guys were going on a 3-month scientific expedition, Penny finally showed interest in Leonard, but kept her feelings a secret. Out of hearing range, Penny admits to herself that she wishes he was going to be gone all summer.

In the third season premiere, Leonard and Penny finally started a romantic relationship kissing him as soon as he returned and making love for the first time. They remain to have a stable relationship and even had the blessing from his mother during her second appearance during episode, "The Maternal Congruence".

The relationship eventually came to an end during "The Wheaton Recurrence" after Penny was not able to tell Leonard that she loved him as well (Although, Penny had been manipulated by Wil Wheaton to end the relationship and rush out, forcing Sheldon's team to forfeit a bowling match.) Leonard and Penny decided during "The Spaghetti Catalyst" that they should stay friends even though they broke up (for Sheldon's sake). Penny's father would later beg Leonard to "not give up on her".

In "The Isolation Permutation", Sheldon became angry when he had to spend the night consoling a heartbroken Amy from being left out of the bridesmaid dress shopping by Penny and Bernadette. He told Howard and Leonard to keep their women in line. Leonard responded that he should not be the person to do that since he was no longer Penny's boyfriend. Sheldon responded that since Leonard was the person who invited Penny to lunch ("Pilot") when she moved into the apartment, everything about her is on Leonard.

In "The Ornithophobia Diffusion", Leonard agreed to go out with Penny to a movie just as friends, their first attempt at going out alone together since they broke up. The first-half of the non-date went well, with the two watching a movie that Leonard picked, a documentary about building a dam on a river in. Penny actually had to pay for her own ticket after Leonard pointed out that they were not on a date and sex was off the table. Later at Brady's Beer they start to argue and interfere with the people they are trying to flirt with. Back at home Leonard told that he did not think they were ready to go out as friends and apologized for his behavior. Penny apologized too, but said that she liked the take-charge and cocky Leonard.

On the spur of the moment in "The Recombination Hypothesis", Leonard asks Penny out on a date after he imagined what getting back with her might be like. Their real date ends successfully and they agreed to take their relationship slow in "The Beta Test Initiation", by treating it as a Beta test and present Bug reports to each other to settle any issues arising.

In "The Friendship Contraction", things seem to move along well with their second attempt at a relationship. During a blackout at the apartment, Leonard decided to hang out at Penny's and they had wine, played with a bubble wrap, and attempted to make out before being interrupted by Sheldon.

In "The Werewolf Transformation", Penny learns the game of chess and beats Leonard the first time she plays. They later picked Sheldon up from Amy's apartment after he went there at 3 a.m. after playing with his bongos in Apartment 4A.

In "The Weekend Vortex", Leonard checks in with Penny whether she would be okay with him spending a whole weekend on a 48-hour Star Wars online gaming marathon. Penny does not seem disturbed by it, which upsets Leonard a little. He doesn't want her upset. Penny asks him what he expects her to say so Leonard tells her that he expects her to at least show that she is upset at the prospect of him being unavailable for the entire weekend. Acting she get upset, then lets him do it realizing that he is a wild stallion that needs to run free.

In "The Launch Acceleration", Penny suggested that they move up their relationship and get intimate; however, Leonard ruins the mood when he asks her to marry him. They later meet and Penny does have the courage to tell him "no" and not break up with him as she did two years previously when he told her that he loved her.

In "The Countdown Reflection", there was some friction between the two of them about the proposal, but when Howard was being launched they were holding hands to comfort each other.

In "The Date Night Variable", Raj, who was butting into their date, insisted that Penny tell Leonard how she felt, but she was not ready and she threw him out of the apartment. Leonard wants to talk to Penny about their relationship, but Penny says that she has had a hard day and wants to just hang out.

Penny is expressing doubts about her feelings for Leonard, though she claims to be happy with the relationship in "The Decoupling Fluctuation". She almost tells Leonard about her doubts and then ends up sleeping with him after looking into his sad eyes. Sheldon's pretty new graduate assistant Alex Jensen flirts with Leonard in "The Higgs Boson Observation", which he doesn't realize, but Penny does take note. When Alex visits and mistake Penny for his lab assistant, Penny takes him back to her place and wants to pull his pants of with her teeth. She doesn't want Leonard to lose interest in her.

In "The Holographic Excitation", Penny revs up their relationship by becoming more acquainted with his work and seducing him in his lab more than once.

In "The 43 Peculiarity", Penny is working with a fellow English student named Cole, who's working with her on an oral class project. Leonard is jealous and follows him down the apartment stairs and tells him to watch out for "Penny's boyfriend" since he was a mean gang member. Penny is upset with Leonard and the two later meet in the hallway and have an argument. Penny tells him that she cannot believe that he cannot trust her and that he embarrassed her in front of Cole who knew who he was since his picture is on her refrigerator. Penny continues to tell him that he is the one she is with and accidentally mentions that he knows that she loves him. Leonard is stunned and tells her that that is the first she ever said that. Penny rushes off to work before she starts crying.

In "The Egg Salad Equivalency", after Alex asks Leonard out to dinner and didn't tell Penny about it, she begins to feel really insecure about their relationship. Leonard apologizes by showing up at her door playing a little ditty on his cello. She decided instead of feeling bad, she would do something about it. First, she looks into science classes at college, but they sound boring. Then she buys some geek glasses to look smart, which does have a positive effect on Leonard as he takes her into his bedroom.

In "The Spoiler Alert Segmentation", Sheldon really makes Leonard mad and he says that he wants to live with Penny and not some crazy person. Leonard suggests to Penny that he move in to which Penny agrees only not to hurt his feelings and even tells Sheldon. She does try to get him admit that Sheldon can't get along without him. Amy plans to move in with Sheldon as roommates. When Amy confronts Penny since she can't move in with Sheldon, Leonard overhears the conversation. Penny tells him that he never asked her if she was ready. Then Amy and Penny go off to drink and talk about their jerky boyfriends and Leonard ends up back in apartment 4A.

In "The Tangible Affection Proof", Leonard takes Penny out to dinner for Valentine's Day. Penny feels that all the efforts never meet the expectations and she will end up disappointed. At dinner, Penny sees an old boyfriend proposing to the girl that broke them up. Leonard again tries to propose and Penny stops him. They begin to argue in front of Howard and Bernadette who have their own issues that night. Later after they barely say good night, Leonard tells Penny that she ruined the whole evening. Penny finally admits that she has commitment issues. She is so happy right now that she knows they are going to eventually get married and since it is forever, that scares her. Leonard then puts the responsibility of proposing on her and that she should do it when she's ready. In the end, she does ask him to be her valentine.

In "The Closure Alternative", Penny was concerned about the passion Leonard showed for science, comics and various television shows while she had no passion in her life. Her dreams had always been to become a famous movie star and live a glamorous life. Anything less than that she felt was settling. Her epiphany comes that she is passionate about her boyfriend Leonard and all her wonderful friends.

In "The Bon Voyage Reaction", Leonard takes a job working on a Stephen Hawking expedition in the North Sea for four months over the summer. Penny will miss him, but knows that it is a good opportunity and sends him off. At the airport, she is fussing over him like a wife sending him off on a business trip. She also has to watch Sheldon over the summer, too.

In "The Hofstadter Insufficiency", Leonard hurts Penny because he seems to be having such a great time on the ship; she thinks that he is not even missing her. In "The Deception Verification", Leonard makes up for it by showing up early and hides in her apartment so that they can spend time together a couple before telling the gang that he's back. It works until Amy and Sheldon find them together after they think that Penny is cheating on Leonard.

In "The Raiders Minimization", Penny purchases a psychology that Beverly Hofstadter has written about all his problems growing up. Leonard is extremely embarrassment, so Penny does whatever he wants to make him feel better including having sex in his lab. He mentions this fact to Howard, so Bernadette tells Penny. She dresses up for sex tempting him and then for revenge makes him Skype with his mother to discuss him manipulating his girlfriend.

In "The Romance Resonance", Howard makes a romantic gesture to Bernadette by writing her a song. Leonard mentions that Penny never does anything like that, except for sex which doesn't count. Penny is determined to romance Leonard. Raj can only suggest that she cook for him or reenact a scene from a romantic movie. Finally after a candlelight dinner, Penny puts on some romantic music, spreads rose petals into her neatly cleaned bedroom and presents Leonard with a first edition of his favorite book from when he was growing up, "". Unfortunately, Leonard had already purchased the book after they had seen it in a used bookstore together. Penny gets upset telling him about all the thoughtful things he had done for her and shows him her collection. An airplane ticket, a rose given to her "just because" and the eleven page letter he wrote her after the first time they slept together. Leonard was very touched that she kept all these things. He then kisses her, showing that Penny did do something romantic.

In "The Thanksgiving Decoupling", Penny reminisces about a Thanksgiving she had in Las Vegas with former boyfriend Zack Johnson which included one of those silly fake weddings. Penny appears devastated when her friends inform her that the wedding was real and she is Mrs. Zack Johnson. Penny and Leonard argue about how quickly she should get her annulment so Penny invites Zack over to their Thanksgiving dinner at Howard's mother. Zack appears as dimwitted as ever telling them that breaking up can be hard on the kids which they don't have. Finally, Penny and Leonard make up and she tells him that the next time she gets married it will be for real, for life and (jokingly) or for money.

In "The Cooper Extraction", Amy points out to the gang that most of them would not know each other if Sheldon had not been part of their lives. Penny feels that Leonard would never have successfully asked her out at the Cheesecake Factory, while Leonard suggests that she would have ended up with Zack. In the Zack story, he spent all the rent money on some magic beans. Leonard might have moved in with Raj; however, he thinks that he would be eating Raj's cooking too much and would be overweight since he didn't have a girlfriend. In Stuart's fantasy, he ends up with Penny.

In "The Hesitation Ramification" and "The Occupation Recalibration", Penny gets a part in the TV series "" and is very disappointed when her scene is cut and not aired. Frustrated and tipsy, she sees her life crumbling when Leonard reminds her that she still has him. She agrees with him and probably to show some advancement in her life, she kneels down and proposes to him. All Leonard can utter is "Umm", much like when he wouldn't succumb to her drunk advances after her Halloween party in "The Middle Earth Paradigm". Penny gets very upset since she doesn't think he wants to marry her. After she storms out, Leonard is afraid to ask her if their relationship is now over. The next day/episode he does visit Penny who assures him that they are still together and that he did the right thing not agreeing to her proposal because she was a mess. Now she has rethought her career and decides to quit the Cheesecake Factory and focus all of her time on acting. Leonard does support her though he is obviously worried about her to give up her only source of income. The proposal count currently is two to one.

In "The Table Polarization", the gang wants to add a dining room table to Apartment 4A which Sheldon objects to. Penny gets Leonard to pick out a dining set that she likes and gets Leonard to stand up more against Sheldon. Sheldon objects to Penny changing Leonard to stand up against him. Penny points out that Amy has changed Sheldon who agrees and then decides to break up with Amy.

In "The Mommy Observation", Penny seems to feel that if she were a successful actress and had to move around, Leonard would leave his job even if it was a tenured position to follow her since she would be making more money. They get complemented when Stuart says that they are best couple he knows.

In "The Proton Transmogrification" Penny asks Leonard to marry her to get their marriage proposal rejection score from 2 to 1 to even. She also gets upset when she keeps getting all the Star Wars in-jokes during their "Star Wars' Day" celebration.

In "The Gorilla Dissolution", Leonard and Penny get back in her apartment after getting fired from her movie. Penny wants to start making smarter decisions about her life. She says that they could get married and Leonard asks her to be serious. Leonard asks her if he is a smart decision, like choosing a healthy boring bran muffin. Penny doesn't care what pastry he is because she is CHOOSING HIM. Leonard is still talking about being a pastry like a strawberry Pop Tart that she is excited about. Penny tells him to forget about it. Leonard does want to marry her, but feels that she is just feeling sorry for herself because she was fired. Penny explains that it may seem that way, but getting fired made her think that fame and fortune is not what is going make her happy. Leonard wants to know what she needs. "You, you stupid Pop Tart!" Leonard finally says that he guesses that he is in, which also bothers Penny. Leonard then retorts that it is an excited "I guess I'm in!" and they agree that they are engaged which seems somewhat anti-climactic. To change the mood, Leonard pulls a ring out of his wallet that he has had for a couple of years, gets down on one knee and asks Penny formally. Penny says "yes" and he slips the ring on her finger that still has ape hair glued to it and they embrace.

In "The Status Quo Combustion", Leonard and Penny announce their engagement; however, the gang is more excited about Raj and Emily hooking up. Leonard goes searching for the missing Sheldon and finds him at the railroad station leaving town to think about all the changes in his life. Leonard wants him to come home, but Penny convinces him that he should let him go.

In "The First Pitch Insufficiency", they go on a double-date with Shamy where Sheldon is bragging about their relationship is so much better than theirs. Shamy has an 8.2 out of ten for relationship stability based upon well proven criteria. Leonard looks at it; however, Penny doesn't want to take the test. She already has his doubts and doesn't need them confirmed. They are engaged and still have very little in common which scares her. It also scares Leonard, but he still wants to face all their problems together. His reassurance really touches Penny and Amy who would like Sheldon to say things like that.

In "The Prom Equivalency", Amy and Bernadette throw a do-over prom on the roof of the apartment building. Penny is at first reluctant to attend the party because they always disappoint her, but Leonard tells her that he really wants to take the most beautiful girl to the prom, this time. She wears her old red prom dress. The roof is decorated wonderfully and Penny really gets into the mood. While others are still getting there, Penny and Leonard dance along without any music on the roof. First they conclude that they probably would not have approached each other if they had been at the same prom. They are both definitely in a romantic mood, while Penny is also glad they aren't having the drama normally seen at proms.

In "The Space Probe Disintegration", Leonard tells Sheldon that his attitude is why he and Penny are not living together. After a tearful period thinking about them him moving out, Sheldon let's him go one night a week to stay at Penny's. Sheldon still sleeps on her couch that night.

In "The Commitment Determination", Sheldon keeps pushing them for a wedding date. The couple insists that they aren't in a hurry, are in a good place emotionally and are concentrating on their careers. Thinking about the wedding details they decide on a small church wedding dressed in black tie tuxes and no releasing of butterflies. Suddenly Penny smiles and sits closer to Leonard. She tell shim that she's free that night. They could go to Vegas and just get married. Leonard agrees. Sheldon is happy for them, but still doesn't want them to sit in his spot. Driving to Vegas, Penny is very happy and glad that they took their time to get to know each other. No surprises and no regrets. Leonard has one regret, from where he was drunk and made out with a woman on his North Sea expedition. Penny was not happy, but still wants to marry him.

In "The Matrimonial Momentum", Leonard and Penny are at a wedding chapel, and decide on their wedding package, which includes a live stream that allows their friends to watch from home. In the chapel, the minister asks Leonard and Penny if they had prepared vows. Penny is surprised that Leonard has and he says that they are made from atoms that have travelled over 14 billion years to create them so they can come together as a couple and make each other whole, which overwhelms Penny. Penny sweetly says that he is the love of her life and her best friend and improvises, quoting the lyrics to '' from the Disney film "". The minister asked if that was from "Toy Story," Penny says it was and Leonard loves it. Leonard gets misty-eyed and confirms that he does. Leonard and Penny are pronounced man and wife. Unfortunately, Stuart and Howard are the only ones remaining who catch it, due to Sheldon insulting Amy, causing her to finally break up with him, causing most of the group to leave. Leonard decides to carry Penny across the threshold into the suite, but huffs and puffs as he carries his wife into the hotel room. They kiss, but Penny still can't stop thinking about Mandy, the girl on the boat. Leonard again insists that the kissing wasn't that great, especially since he also chipped her tooth. Penny wants to know if he feels guilty. Leonard jokingly says that he does not feel guilty because the girl had dental insurance. Leonard then adds that he does feel guilty when he sees her every day at work, freezing when he quickly realizes his mistake. Penny is now truly shocked that he never told her this fact. Instead of spending the night in Vegas, they return back to their apartments, still fighting. Leonard sadly spends his wedding night with Sheldon, who isn't surprised at all.

In "The Separation Oscillation", Leonard has a nightmare that Penny got even with him by making out with Sheldon. This spurs Leonard to try to make things right. He finally confronts Mandy, his kissing buddy from the North Sea, about the incident and it doesn't seem to mean much to her. Leonard also realizes that he really was trying to sabotage the relationship. He talks with Penny and tells her. She too realizes the she has been trying to sabotage their relationship. Leonard then confesses, "I've loved you from the moment we met and I will keep loving you until the end of time." Penny is very touched and tells him that that was the most beautiful thing anyone has ever said to her. He follows with getting down on one knee and asks, "Penny Hofstadter, will you please stay married to me?" She accepts and they reconcile, kiss and head into the bedroom to formalize their marriage.

In "The 2003 Approximation", Leonard finally moves out of his apartment and eventually moves in with Penny into her apartment. The sleeping arrangements are still a little complicated. Even though they are living together in 4B, they sometimes sleep in 4A, in Leonard's old room. This is due to Penny having a change of heart at the last minute after feeling sorry for Sheldon who was having a hard time dealing with his breakup with Amy and Leonard moving out. In "The Helium Insufficiency" it is also apparent that Leonard is still maintaining an office in 4A.

In "The Valentino Submergence", Leonard and Penny go out to dinner for Valentine's Day but the wait is at least one hour and they suffer through an unsympathetic maitre d'. Penny's decision to flirt with him gets nowhere and she gets called "ma'am". She tries to get him to acknowledge that they are almost the same age; however, he is 21 and she is 31. Next she drags Leonard out of the restaurant because she is still young. After a fast food meal in the car, they look for something young and fun to do. The "Moulin Rouge" movie is too late, no skinny dipping or dancing either. They end up dressed as cupids breaking in on Sheldon's "Fun with Flags" programs yelling "Happy Valentine's Day" and that they are "young and fun".

In "The Positive Negative Reaction", Leonard and Penny talk about having children one day. Penny says she is not in a rush, but someday they can. Leonard adds that he thinks they would make amazing parents.

In "The Big Bear Precipitation", while spending the weekend in a cabin, Penny learns that Leonard has been keeping a secret bank account for emergencies because he doesn't feel that Penny can handle money due to her large credit card debt. Penny, who makes more than him, admits that she does not like her job having to flirt to get her sales; however, she will keep at it because it is the responsible thing to do.

In "The Viewing Party Combustion", Leonard gets tired of the quarterly Roommate Agreement meetings where nothing important is ever discussed. Penny who has signed the latest one as an occupant of the apartment and wants to attend since she gets to lead them in the pledge. Leonard sulks about attending the meeting which makes Penny mad driving her and Sheldon over to her apartment breaking the group into two separate camps. Leonard also points out the weird brother/sister relationship she and Sheldon has which Amy is also jealous of. Penny often takes Sheldon's side due to her sibling feelings for him.

In "The Convergence-Convergence" Leonard has invited his dad to his and Penny's wedding re-do and has to deal with his divorced parents. He just wants everyone to be civil. After delivering a toast to Penny thanking her for a wonderful year of marriage, his father exasperated with his ex-wife leaves with Mary Cooper. They find a connection in their mutual hatred of Beverly and appear to hook up much to Sheldon and Leonard's discomfort, meaning they could end up being step-brothers.

In "The Conjugal Conjecture"; the actual ceremony is officiated by Bernadette. Penny tells him how much she loves him and how glad they did elope. Leonard just tells her how much he loves her and that he can't believe she is with him.

In "The Cohabitation Experimentation", Sheldon and Amy move into Penny's apartment for a five week cohabitation experiment after Amy's apartment suffers a plumbing problem. The Hofstadter's are so excited about finally having a place of their own that they put on music (which Sheldon would never let them do) and have a private dance party in their underwear.

In "The Fetal Kick Catalyst", Penny is signing autographs for fans of the Serial Ape-ist movies where the fans think that she is a terrible actress and can't believe that the nerd Leonard is her husband. The fans sit around Leonard as he tells his story how he won the hot actress.

In "The Veracity Elasticity", Leonard learns from Howard that Penny has secretly been moving his collectibles into storage. Although he's mad that she doesn't feel guilty about it, he does understand that she really hasn't been feeling comfortable like she belongs there. He allows her to redecorate their room any way she wants it. Upon seeing Penny's new design for the bedroom, he immediately hides some of her stuff in her old closet.

In "The Birthday Synchronicity", Bernadette has a false alarm trip to the hospital before she has her baby interrupting both Lenny and Shamy who are trying to have sex.

In "The Holiday Summation", the Hofstadter's cheerily head off to cut their first live and have a horrible experience. Leonard lets go of the sending it flying, the tree slips of the  roof while driving it home, Leonard collapses under the tree while carrying it up The Stairs and after getting it into the apartment, they find an animal lurking in the tree. The tree ends up being shoved down The Elevator shaft.

In ”The Romance Recalibration”, Penny feels that Leonard is no longer trying in their marriage; specifically he sits around in his underwear playing video games and never brings her flowers. Leonard thinks that he is the only one in their marriage that has ever worked at it. Leonard follows Penny to the spa that she took Amy instead of him and they decide that they do have some challenges in their marriage. Reluctantly, both ask Sheldon to prepare them a Relationship Agreement like his and Amy especially since he and Amy appear to be so happy living together.

In "The Emotion Detection Automation", Leonard and Penny have an argument over her brother Randall coming to Pasadena and staying with them, an argument where Leonard's friendly, reasonable and thinking-carefully personality goes the other way around.

In "The Escape Hatch Identification", Beverly thinks Leonard and Penny are trying to keep their "terrible" marriage intact since they took Raj in for a while and also asked Randall to live with them, which is not true since Leonard and Penny are okay in their marriage and are not looking for roommates, but are instead being generous to let someone they know quite well to live with them, until they can get a house of their own.

In "The Proposal Proposal", Bernadette suggests that Leonard and Penny should have a baby in order to try and calm Howard and Bernadette's worries about having another baby so soon, but Leonard and Penny say "no". They also celebrate their second wedding anniversary, which they'd forgotten about.

In "The Bitcoin Entanglement" (S11E09), Leonard and Penny visit Zack to reclaim Leonard's old laptop that Penny had given him years ago. Zack also shows them an old video a drunk Penny made after she broke up with Leonard seven years before. She did love him, but was scared of a permanent commitment. They are happily married and are best friends

In "The Conjugal Configuration", Larry Fowler is hiding out in Sheldon and Amy's apartment. Mrs. Fowler comes over demanding that her husband return. He decided to take a break from their marriage. His wife comes over to talk things over with Penny and decides that she is a good person and they should become friends. Penny marches over to 4B and demands that Larry go home. Penny gets mad at Leonard when he compares their relationship to the bickering Fowlers, but she lightens up when he re-evaluates what he said.

In "The Procreation Calculation", Penny declares that she doesn't want to have children because their life is so great as it is. Leonard is disappointed and then declares that that is one thing on his bucket list that he'll never do like when he wanted to own a Batmobile. In the end Penny does rent him a Batmobile for the day.

In "The Imitation Perturbation", Leonard reminds Penny of their first kiss on their couch during her Halloween party as they host one for the gang. Penny can't remember it until she admits that she does, but she was drunk and mad at Kurt. She would rather remember their first official kiss as at Leonard's birthday party where she gave it freely. Leonard agrees that that will be their first official kiss.

In "The Grant Allocation Derivation", he has to distribute some excess grant money which he takes very seriously. Penny finds that very sexy until Leonard can't decide between the last three.

In "The Propagation Proposition", Zack and his new wife Marissa want to have a baby, but Zack is infertile. They decide to ask Leonard to help them out since he thinks he's nice and smart. Leonard said yes and wanted to help them out. Penny was against it and then said that he could do it since she had decided to not have any children without considering what Leonard thought about it. Leonard at first was willing to help them out until he decided he didn't want a child in the world that he and Penny weren't going to raise ("The Donation Oscillation"). Also, Penny realizes with her father's help that she's doubting her "no child" decision, especially when she's under the impression that she's letting everybody down.

In "The Decision Reverberation", Penny again tells Leonard that he should stand up for himself more. Leonard decides that he wants to be the principle investigator on a plasma physics study so he prepares a proposal and plans to threaten to quit if the President Siebert will not accept his proposal. Penny is worried about him quitting because she doesn't want to move. Even though President Siebert turned him down, Leonard does get called back and offered a different job as co-lead on a photo entanglement project. Leonard is happy and Penny is very proud of him.

In "The Change Constant", the Hofstadter's and the Cooper's are waiting for the Nobel Committee to call from Sweden. Kripke gives them a fake phone call calling them suckers. Amy then gets the call and quietly says that they won. Sheldon can't believe her so Leonard slaps him, then they all shout excitedly and have a big hug together. Leonard had had permission to slap Sheldon if he fell asleep, but that didn't happen earlier. Sheldon has a hard time dealing with the fame and attention from his colleagues and reporters. Then after Raj gives Amy a makeover he runs off with Penny to The Cheesecake Factory cursed by all the changes. Penny reminds him how much he's changed in the last twelve years and that the fact of change is really the constant. Sheldon accepts that.

In "The Stockholm Syndrome", Leonard and Penny are keeping a secret that she is expecting. When Penny returned from drinking with Sheldon at The Cheesecake Factory, they hopped into bed without "wrapping" something. Penny seems happy, but is unsure about the whole baby thing, while Leonard is ecstatic that he's going to be a father. Sheldon and Amy pay for their clothes, ticket and hotels so they can all attend the Nobel Prize ceremony. Sheldon notices that Penny was quite sick on the plane, so the secret comes out and he is just glad he won't get sick. Leonard is mad that Sheldon doesn't congratulate them and the Hofstadters say they are heading home. Penny is very sick and claims that she can't travel and has to be there for Amy, with Leonard doing the same for Sheldon. At the ceremony Sheldon puts aside his self-centered speech and realizes that his achievement isn't just his, but also his family and friends who encouraged him and tolerated him. Sheldon introduces his friends to the world and tells him how much he loves them.

Elizabeth Plimpton
Dr. Elizabeth Plimpton is a colleague of Sheldon's who stays with Leonard and Sheldon when she visits Caltech while looking for a job. The first night, she seduces Leonard before attempting to set up a gang-bang with Leonard, Howard, and Raj. Raj gets rid of the other two. Leonard did not pursue a relationship past the one-night stand.

Leslie Winkle
Leslie Winkle is a fellow scientist who works in the same lab as Leonard and is very much like Leonard. The first time Leonard asked Leslie out, he was rejected. His further relationships with her proved to be only short-lived casual sexual intercourse. Prior to his later encounters with Wil Wheaton, Sheldon considered Leslie his archenemy and one of their disputes resulted in the final breakup between Leslie and Leonard after Leonard supported Sheldon's theories over her own. After learning from a drunken Penny that friends can have meaningless sex he approaches Leslie. She rejects him immediately. They meet up again at Sheldon's birthday party and she congratulates him for his marriage to Penny and she says that she thought he'd be living with Sheldon forever, although Leonard indirectly points out that he in fact still does.

Stephanie Barnett
In The Lizard-Spock Expansion Leonard was indirectly introduced to Dr. Stephanie Barnett by Howard, who was interested in him. Howard had picked up Stephanie by inviting her to a drive a car on Mars, a pick-up line that he had also tried to use once on Penny. After getting the Mars Rover stuck Howard called Leonard and the rest of the gang to help him. Leonard's job was to take Stephanie home because, "She didn't exactly have clearance." In the next scene Leonard and Stephanie are making out while trying to tell her how great a guy Howard was. Stephanie agrees while continuing to make out. Finally Leonard asks if Howard ever really had a chance with her. Stephanie assures him the Howard never had a chance. Leonard asks if they can see each other again. Stephanie informs him that he had not finished seeing her that evening.

In The White Asparagus Triangulation Sheldon is quite taken with Stephanie, because she feeds right in to his hypochondriac personality and would make a great medical officer for the "Landing Party." He thinks Leonard does not know what he is doing in the conduct of their relationship so he tries to help. Leonard tries to ditch Sheldon by taking off to see a movie, but fails. Sheldon tries to "relieve Leonard of command" under Star Trek General Article 104 section 2 leaving Leonard to point out that General Article 104 does not apply here because this is not "Star Trek". He then ends up getting Leonard to cut his hand very badly trying to demonstrate Leonard's dominance as the "Alpha Male" by opening a jar of asparagus Sheldon could not open. Stephanie has to perform her Dr. duties and sew up his hand. The episode ends with Leonard's Facebook status changed to "in a relationship" by Sheldon. Howard tells him that he looks needed; however, Stephanie responded by changing her status to "in a relationship" with Leonard.

In The Vartabedian Conundrum, the relationship is going well. Sheldon give Leonard some papers for Stephanie to sign in regards to the implementation of the live-in girlfriend section of the Roommate Agreement. Leonard insists that they are not living together, but takes the papers anyway just to get Sheldon off his back. The following morning while all are getting ready to leave, Penny comes over in very skimpy PJ's. There is a very embarrassing situation when Stephanie meets Penny for the first time Leonard tries to explain who she is and why she has a key to their apartment, and why she is here in her "underwear." Penny is quite amused while Sheldon's comments don't help. Only half satisfied, Stephanie leaves for work. Responding to Sheldon's declaration that Leonard and Stephanie are living together, and Leonard's denial, Penny sets out to help determine if in fact Stephanie is living with them. She inspects his bedroom leaving Leonard to finally admit that they are living together. This creates a panic in Leonard because he feels things are moving too fast for what he is ready. They eventually broke up, but the details of their breakup are unknown.

Mrs. Latham
Mrs Latham appears in "The Benefactor Factor" and is played by actress Jessica Walter. The gang is attending a Saturday night fund raiser for Cal Tech and they meet Mrs. Latham, a major university donor. She likes to throw zingers at people and make them uncomfortable, which she considers an advantage to being rich. Mrs. Latham invites Leonard to dinner to talk more about his research. After the dinner, she was flirting with Leonard on the way home. She kisses him on the lips, causing great discomfort towards Leonard. The next night, Mrs. Latham asks Leonard if he's hungry. He replies with "For food, right?", indicating indirectly that he is uncomfortable. She picks up right away; however, she tells Leonard that she was going to donate the money to Cal Tech whether anything happened or not and hints to Leonard that she is skilled enough in the art of love-making to land her rich husband. Once Leonard realizes that he is not just being used, he gives in. The following morning, Leonard gets home just as Penny is going out for a jog. She kids him about doing the "walk of shame".

Priya Koothrappali
Priya is Raj's younger sister, she was first introduced to the gang on April 12, 2005, on that day Leonard was able to establish a sexual relationship with her despite his pact to not do so with Howard. Their relationship was not known by the gang for five years, where Leonard continued his sexual relationship with Priya. By this point, Leonard wanted to proceed with their relationship, even suggesting moving to India with her, but Priya had no interest in proceeding.

In "The Cohabitation Formulation", Priya and Leonard discussed their relationship and decided to start dating each other, much to Raj's dismay. In "The Roommate Transmogrification", it was revealed that Priya was moving back to India. In spite of the physical distance between them, Priya and Leonard are at present attempting to maintain a long distance relationship, communicating via Skype.

Leonard and Priya's relationship seems to have come to an end after Leonard admitted to making out with a comic book artist and admitting it to Priya and apologizing, Priya then admitted she slept with her ex-boyfriend. Leonard then closed the laptop and he claimed he was single in "The Isolation Permutation" after leaving Sheldon at Amy's apartment.

Alice
Alice is a girl who meets Leonard at the comic book store and then seduces him. This incident occurs during the period when Leonard is having a long distance relationship with Priya. Leonard initially gives in to temptation, but at the last minute pulls away. She calls him a jerk. Next time he's communicating, via the webcam, with Priya, he confesses his weakness. To his surprise, Priya has no problems with it. Upon questioning, Priya reveals that she has not been faithful to Leonard, resulting in them breaking up.

Alex Jensen
Alex Jensen is a Caltech physics doctoral student and Sheldon's new research assistant. Alex first triggers Amy's jealousy; however, when she and Penny see her flirting with Leonard, Amy says that Alex is Penny's problem. When Penny introduces herself, Alex mistakes Penny for Leonard's assistant. Penny says that they were working on a five year project. Alex replies that she is sure that a lot of people would want to work with someone as talented as Leonard. Penny quips that a lot of people can't and then she takes Leonard to her apartment for bedroom activities. As Penny is leaving with Leonard, Alex remarks that she seems nice sounding like she has no ulterior motives outside of working with Sheldon. Alex is next seen in "The 43 Peculiarity" talking to Raj and Howard. In the Caltech cafeteria she spots Leonard who tells her to call him "Leonard" and not Dr. Hofstadter. He discusses his jealousy of Penny's English classmate whom he thinks is hitting on her. Alex says that it's probably nothing and that he probably gets hit on all the time. Leonard laughs it off, though she tells him that he probably gets hit on without even knowing it. Leonard doesn't pick up on her signals that she really likes him. He leaves as she looks a bit depressed. Later she texts him that they can get together and talk any time he wants. Leonard thinks that she is just being friendly.

In "The Egg Salad Equivalency", Alex asks Leonard out to dinner after he expresses an interest in the Physics lecture she is going to and this request really startles Leonard. Though it makes him feel good "to the bones" to have an attractive young woman interested in him, he tells everyone he still loves Penny and wouldn't do anything about it. He mentions Alex's actions to Sheldon who tries to talk to her about it and lands the whole gang in the Human Resources Administrator's office. He also didn't tell Penny about Alex's actions, which made her feel really insecure about their relationship. Leonard apologizes by showing up at her door playing a little ditty on his cello.

Other mentioned girlfriends

 * French woman who had a PhD in French literature.
 * Karen Berberick - High school crush. Leonard describes their relationship as moving so slow, she didn't even know they were dating.
 * Geraldine Coco - Not technically a girlfriend (as far as we know), but she was the one who gave Leonard his first kiss when he was 19.

Trivia

 * Sheldon is more popularity than Leonard with the fans; however, in his world, people like him more than they like Sheldon, which is understandable considering their personalities.
 * His character has appeared in every episode including both pilots.
 * Leonard wants to achieve a Nobel Prize someday.
 * Leonard is skeptical of loop quantum gravity.
 * While Leonard's IQ is never directly stated in the show, he states in the "Pilot" that he and Sheldon have a combined IQ of 360. Sheldon states that his IQ is 187, so therefore Leonard must have an IQ of 173. Sheldon also states his intellect cannot be accurately measured by a "regular" IQ test.
 * IQ scores are an expression of what percentage of the population a person is "smarter" (achieved higher result) than. An IQ of 100 is assigned to the mean population score ("average intelligence") and a point of IQ is defined so that one standard deviation of the sample corresponds to 15 points (on modern tests; older tests like the classic Stanford-Binet Test had 16 points per st. deviation). Tabulation of IQ score rarity is available online; using that, an IQ of 173 occurs approximately once in 1.75 million individuals (assuming the score comes from a modern SD-15 test), and there are therefore some 4,150 people in the world (assuming a world population of 7.25 billion) at least as smart as Leonard (or in other words, Leonard is smarter than all but 4,150 people in the world). If instead Leonard scored 173 on an SD-16 test, then his intelligence is about 1 in 400,000, i.e. there are some 18,000 people in the world at least as smart as him.
 * On the other hand, it is true that all current IQ tests are imprecise for very high (or very low) scores—this is because there are very few people with such scores, rather than being due to bad design of the tests themselves.
 * Leonard received his PhD at age 24.
 * Leonard is 5 ft. 5 in. (1.65 m) tall – the height of Johnny Galecki. In "The Egg Salad Equivalency", Sheldon states his "short, bespectacled colleague" is "... strutting around like he’s five-foot-six," which implies that Leonard is shorter than this measurement. (Refer to the Trivia of "The Decoupling Fluctuation".)
 * Despite being short, Leonard states he is "too tall to be a jockey" (Though to Sheldon's amusement he's the perfect person for the sport).
 * Leonard is a quarter inch taller than Howard.
 * By later seasons, Leonard's height doesn't appear to be too much of an issue for him anymore, given that he's now been shown to be taller than Amy, and roughly the same height as Penny.
 * In "The Alien Parasite Hypothesis", which aired in 2010, Leonard says, "12 years after high school, and I’m still at the nerd table." This fact implies he was born in 1980, the same year as Sheldon, provided the following assumption that he graduated at the usual age of 18-years-old is supported by him having had a senior prom, which is mentioned in "The Peanut Reaction".
 * Leonard has a grandmother with Alzheimer's that came to Thanksgiving and stripped naked, carving the turkey prior to the pilot.
 * Leonard wears boxer shorts rather than underpants or trunks.
 * Leonard always wears neutral colored sweat jackets.
 * Despite his lactose-intolerance; however, Leonard can actually process small amounts of non-fat ice-cream without farting (according to Sheldon).
 * Leonard has Darth Vader 'No More Tears' shampoo and Luke Skywalker hair conditioner.
 * Leonard is a huge Superman fan, so much so that he owns all of the Superman films and more than 2600 Superman comics, as well as Superman underwear. He also has an unseen Superman costume with new boots that adds three inches to his height, which he wore at the comic book store's annual New Year's Eve party when the group went as the Justice League. Sheldon says he is their weak point, and he got replaced by Penny's friend Zack at a later party.
 * Many of Leonard's shirts are scientifically inspired, such as in episode 1x03 where he can be seen wearing a shirt with a "Paradox" molecule.
 * Leonard and Sheldon own Dell XPS computers. After "The Bozeman Reaction," he owned a Dell Adamo XPS.
 * Leonard's favorite TV shows are Babylon 5 and Buffy the Vampire Slayer
 * Leonard can play the cello, mirroring Johnny Galecki's own talent of playing the instrument.
 * Leonard used to get stuffed in his own cello case.
 * Leonard and Sheldon are named after actor and producer Sheldon Leonard.
 * Leonard is unaccustomed to observing birthdays, by choice of his mother's views on psychiatry. He had never had a birthday party before moving to Pasadena.
 * Leonard's parents both formerly lived in New Jersey. His father now lives in New York after his divorce.
 * Leonard drove a silver Volkswagen Touareg with a luggage rack on top early on in the show.
 * Leonard plays lead guitar on Rock Band.
 * Leonard plays Age of Conan with the guys and is probably an Assassin (his character can be seen when he is checking on Penny via the game).
 * Leonard has sleep apnea.
 * Leonard gets carsick unless he sits in the front.
 * Leonard has trouble making eye contact, only being able to do so with Penny and the rest of his friends, which could be indicative of Asperger's Syndrome, ADD, or simply social anxiety.
 * Leonard is the group's (not including Penny) Wii bowling 'Team Captain'.
 * Leonard's last name (Hofstadter) is in honor of Nobel Prize Winning Physicist Robert Hofstadter, who worked at {{w|Stanford University in the field of Nuclear Physics.
 * Leonard dislikes his name since it has "Nerd" in it, saying he prefers the name Angelo as it has "angel" and "Jello" in it, although he was high at the time.
 * Leonard claimed he could hear Penny flushing her toilet from his apartment.
 * Leonard tried to get the nickname "Duncan" by carrying around a Duncan brand yo-yo. This only resulted in bullies stealing the yo-yo and stuffing their socks into his mouth, making him "Sockmouth".
 * Leonard was his school's debate club president.
 * Leonard believes he is the "King of foreplay", as he used to do lengthy shows for Penny before they had sex.
 * Leonard gets asthma attacks when he has panic attacks.
 * He and Bernadette are the only main characters who wear glasses.
 * Leonard tried wearing contacts in 7th grade, but could never get used to them.
 * Leonard wears black framed glasses (he originally wore tortoise shell glasses eight years ago), even if in costume.
 * He is the only character who has had five girlfriends Joyce Kim, Leslie Winkle, Penny, Stephanie Barnett and Priya Koothrappali), followed by Howard with four girlfriends (Christy Vanderbel, Leslie Winkle (friends with benefits), Lisa and Bernadette Rostenkowski) unless their are others that are not referred to.
 * He is afraid of horses.
 * Leonard always wears shorts when he goes to Kurt's apartment for Penny's errands.
 * Leonard is usually wearing a t-shirt when he is in bed with Penny though not he first time.
 * Also, until "The Colonization Application", he kept his socks on.
 * Like Sheldon, Penny, Rajesh, and Amy, Leonard became a reverend to marry Howard and Bernadette.
 * Bernadette, Raj, Leonard, Penny and Amy are the only members of the social group/main cast whose parents have both been seen on-camera - Sheldon's father died when Sheldon was a child, Howard's father abandoned him and his mother when Howard was eleven, Stuart's mother and father have been mentioned and are both alive, Emily's father has been mentioned and it is not known if he is dead or alive and there has been no appearance or mention of Emily's mother or Leslie's mother and father to date.
 * Leonard's great-great grandmother was half-Cherokee (as stated in "The Irish Pub Formulation"). This statement makes Leonard 1/32nd Cherokee.
 * Estimated by Howard; Leonard's ideal mate is a short, low self-esteemed woman scientist living in a government facility in
 * Throughout his childhood, his mother constantly conducted tests on him.
 * As he stated in "The Contractual Obligation Implementation," when he was in middle school he wanted to be a rap star like Soop Dogg, but with "a healthy respect for the police." He states his parents pushed him into becoming a scientist.
 * Though, after watching Professor Proton, he eventually became inspired to follow through with science.
 * One of Leonard's favorite fictional characters is Frodo Baggins from "The Lord of the Rings" (as stated by Sheldon in "The Large Hadron Collision").
 * Leonard finds himself dating a married woman in "The Thanksgiving Decoupling".
 * The final proposal count to each other was Penny 3; Leonard 3.
 * It has been suggested that the character of Leonard Hofstadter owes something to that of Professor Ponder Stibbons, a Research Wizard in Sir Terry Pratchett's Discworld series of fantasy novels. Ponder, in a university of Wizards devoted to magic, takes a minority view that Science is also important; he is to all intents and purposes a physicist. Leonard is a research physicist who enjoys fantasy works and plays games where he role-plays a Wizard. (Mystic Warlords of Ka'a). Both are geeky young men with glasses who affect clothing which is shapeless, baggy, neutrally colored and for preference has a hood. The Science of Discworld series, which are co-written with eminent British scientists who are well known in USA academia, use Pratchett's fantasy world as a means of explaining ongoing scientific thought to a lay audience. The Discworld story begins with a massive explosion of magical energy that has to be diverted down a safe path, in order to avert a seriously Big Bang. Ponder Stibbons discovers he has inadvertently created a whole Universe out of nothing. This cools and expands and passes through stages like drooling autotrophs (explicitly referenced) until, presumably, California and Caltech evolve. It is possible Terry Pratchett watches and enjoys TBBT, and/or that Chuck Lorre reads Discworld novels.
 * Leonard's relationship with Penny is the only one on the show that isn't set up by someone. Howard and Bernadette were set up by Penny, Sheldon and Amy were set up by Howard and Raj through a dating website, and Raj met Emily on a dating site. Stuart actually hired met his girlfriend and assistant, Denise, to work in his store.
 * Leonard and Sheldon are similar of the characters of "The Odd Couple". Leonard Hofstadter is similar to Oscar Madison. Leonard may not be a slob, but he is funny loving and nice. He is really cool and childlike.
 * Leonard would just settle for being nominated for a Nobel Prize.
 * Sheldon and Leonard drive up to Skywalker Ranch to see if they could get in. When they can't, Sheldon runs through the gate and almost gets the two of them arrested for trespassing. Despite not actually running with Sheldon, Leonard is permanently banned from Skywalker Ranch along with Sheldon, and will be arrested if he ever comes back.
 * Leonard, Sheldon, Howard and Raj are the only characters of the current main cast who have appeared in every episode. If the unaired pilot counts, then only Leonard and Sheldon appear in all episodes.
 * Leonard is right-handed.
 * Before the episode "The Peanut Reaction", Leonard had never had a birthday party.
 * Leonard's iPad password is his birthday, specifically 4 digits the month and day, according to him, and since Leonard's birthday is known for being May 17, that would make his iPad password 0517.
 * Despite the fact that Leonard's password for everything is "Kal-el", according to Sheldon.
 * Leonard's go to when he's in bed with a girl is "I'm sorry."
 * In the episode "The Staircase Implementation", Leonard states that with Star Trek he prefers the original series over the next generation, but he prefers Picard over Kirk.
 * He nicknamed his genitals "Alvin and the Chipmunks".
 * It's strongly implied that Leonard resents Zack.
 * He drives a Silver Saab Sedan 2002
 * His life would be ruined if it wasn't for Sheldon as he could be end up in jail for leaking confidential information to Joyce Kim or lost his life from his rocket fuel experiment.
 * Leonard shares some similarities with fictional character Sherlock Holmes in that while they are both geniuses, they are also the least intelligent person in their respective families. Also, they are both the middle child and have one brother and one sister.
 * As evidenced in The Skank Reflex Analysis and The Ornithophobia Diffusion, Leonard has a taste for Newcastle Brown Ale.
 * Things in common:
 * They each have one brother and one sister.
 * They are the second born child of their respective parents (possibly).
 * They had issues with the opposite-gender parent while growing up.
 * They are the love of each other's life
 * Their careers are in some ways similar; While Leonard is a smart, brilliant and successful scientist and physicist and well-educated and accomplished academic, his work in experimental physics somehow appears to mirror Penny's average education status and her pharmaceutical sales career in which she has often had to flirt with doctors to make sales. This makes it similar due to Leonard's work being sometimes derivative and his friends having more accomplishments, as well as him being the least successful of his family.

Gallery
Leonard Hofstadter/Gallery