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 both that apartment and in Apartment 4B. 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 the group. The writers have toyed with a romance between him and neighbor Penny, with their on-and-off relationship being a major force for drama.

Early Life (Before the show)
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 The Hulk, using his mother's make-up and made-up muscles (balloon muscles) with a bra of his mother's to stop them 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 bottom with Leonard's brand new ping-pong paddle and Leonard thought it was a dream until years later ("The Raiders Minimization", S7E4). 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 parents' wish to study the holidays for their "anthropological and sociological implications on society." ("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. 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 cello, "getting me beaten up enough", as he described himself ("The Hamburger Postulate", S1E5). Johnny does play the cello.

Season 1
Leonard is introduced living with Sheldon Cooper and when they return home, they meet Penny, their new neighbor. She has moved into the apartment across from theirs because she has broken up with her boyfriend Kurt. Leonard instantly falls in love with her, even inviting her to their apartment for lunch. The boys are shown to be friends with Howard Wolowitz and Rajesh Koothrappali. She takes the boys to get her TV from Kurt, but he steals their pants and they return home without them. Penny apologizes for putting them through it and later she and the four guys go out for dinner (Pilot). In the last episode of the season (The Tangerine Factor), Penny is distraught about a breakup with her boyfriend because of him blabbing about their sex-life online, she laments to Leonard that for once she just wants to date a guy who is nice and cares about her. Seizing the opportunity, Leonard asks her out, and she accepts. The season ends with them going on their first date.

Season 2
His first date with Penny went well, but they broke up shortly afterwards. He goes on to date Stephanie and Leslie, but both 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. One night, 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.

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 towards 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 apartments. (The Matrimonial Momentum). Leonard eventually 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 being there for them.

Career
Leonard is an 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 when mixed into Tovex in order to create a combustible gel. 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. 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 Italians. 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 latest 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 Netherlands. 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 North Sea 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 George Lucas' Skywalker Ranch headquarters.

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 group, and is quick to try new things apart from his geek friends.

However he does display selfishness, normally putting his needs before his friends well being. During the group's expedition to North Pole 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 and Howard and Raj were irritated by Sheldon and when they return from the North Pole he is totally humiliated among his scientific peers. When 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 sex with Penny. Another example of Leonard's occasional selfishness was when he was sleeping with Raj's sister Priya, and didn't seem to care how Raj felt about the situation.

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 demoralises 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 Sheldon's fault). He also admits to Sheldon that he is a narcissist who needs his constantly battered ego nourished from people who are much less smarter than he is while presenting their paper to a class of grad students.

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).

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 light saber 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 ten 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 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, Babylon 5, 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. 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 Louis Leakey). 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 or he 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 8th 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.

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 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.

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 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.

Leonard's brother, Michael, is a Harvard law professor and is engaged or 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 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.

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.

Family Members

 * Father: Alfred Hofstadter
 * Mother: Beverly Hofstadter
 * Brother: Michael Hofstadter
 * Sister: Unnamed Sister
 * Brother-in-law: Unnamed
 * Uncle: Floyd Hofstadter
 * Aunt:  Edna
 * Cousins: Unnamed
 * Grandfather: Unnamed
 * Grandmother: Unnamed
 * Wife: Penny Hofstadter (née ?)
 * Father-in-law: Wyatt
 * Mother-in-law: Susan
 * Sister-in-law: Unnamed
 * Brother-in-law: Randall
 * Brother-in-law: Unnamed
 * Nephew: 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 Department of Homeland Security. 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 Kryptonite. 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 George Lucas' Skywalker Ranch 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.

Howard
Howard meets Leonard in a flashback in "The Staircase Implementation". Howard told Penny that both he and Raj liked Leonard. 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!").

Raj


Leonard and Raj meet at the University in a flashback in "The Staircase Implementation". 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 Switzerland 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 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.

Amy
Amy and Leonard did not get on very well at first, 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 parking lot in front of the liquor store 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 let 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 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.

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 are 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 genius 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.

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.

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 semi-serious 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 North Korean spy attempting to get information on the 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." 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. 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. In the third season premiere, Leonard and Penny finally started a romantic relationship, and made 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 Will Wheaton to end the relationship and rush out, forcing Sheldon's team to forfeit the game.) 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 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 South America. 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.

The two later went to Brady's Beer and while Leonard went to get them fries (which Penny had to pay for hers since Leonard did not want her to eat his fries), Penny started flirting with a geeky screenwriter named Kevin. A ditched Leonard started flirting with a woman named Laura, before Penny suddenly interfered. The two ended up saying horrible things to each other to ruin the other's attempt at flirtation, such as Leonard owned two Star Trek costumes that he wore even when it was not Halloween and Penny thought that Cold Wars were fought only during the winter. They ended the night walking back to the apartment, both looking gloomy. 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. She said goodnight and went to her apartment.

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. with his bongos.

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. She tells him that he does not need to check with her for such matter. He, on the other hand thinks that he should, since he does not want to upset her, yet if he does check in with her, he does not want her to think he is being too serious, since they are still at the Beta Test phase of their relationship.

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, but realizes that he is a wild stallion that needs to run free. Penny tries it again with the version that Leonard wants and the two share a kiss.

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 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. 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 respond to, but Penny does take note. She is working up to telling him that she loves him. In "The Holographic Excitation", Penny revs up their relationship by becoming more acquainted with his work and seducing him in his lab.

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 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.

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. He asks her if she's all right with it, but she only agrees 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 over hears 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 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 his 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. When he was six, Leonard walked in on his naked parents while his mother was spanking his father with his new ping-pong paddle. Leonard had thought that that incident had been a nightmare of his youth. 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, The Hitchhiker Guide to the Galaxy. 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 "The Hesitation Ramification" and "The Occupation Recalibration", Penny gets a part in the TV series NCIS 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. 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.

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-climatic. 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 time 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 doubt 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 want to face all their problems together. His reassure 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 close 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 wad 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 'You've Got a Friend In Me' from the Disney film "Toy Story". 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 went and talked 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 get tired of the quarterly Roommate Agreement meetings where nothing important is ever discussed. Penny who has signed the latest on 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.

Elizabeth Plimpton
Dr. Elizabeth Plimpton is a colleague of Sheldon who stays with Leonard and Sheldon when she visits Caltech for a conference. 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. 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 her. 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. Leonard is 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," but 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 changing his status to "in a relationship" with Stephanie on Facebook. This was of course a hacked entry by Sheldon. But Stephanie responded by changing her status to "in a relationship" with Leonard. In The Vartabedian Conundrum, the last episode in which we see Dr. Stephanie, 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, buts 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." 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. 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—why and when—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. 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
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 Relations Administrator's office. He also didn't tell Penny about and Alex's actions make 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 Berbericky - High school crush. Leonard describes their relationship as moving so slow, she didn't even know they were dating.

Trivia

 * Despite being the main character, Sheldon has more popularity than him. However, in his world, people like him more than they like Sheldon.
 * 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".)
 * 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. The 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.
 * Leonard wears boxer shorts rather than underpants or trunks.
 * Leonard always wears neutral colored sweat jackets.
 * Leonard wears black framed glasses (he originally wore tortoise shell glasses eight years ago), even if in costume.
 * 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 the most recent party.
 * Sheldon reveals that for everything his password is 'Kal-el'.
 * 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 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 live in New Jersey.
 * 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 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 tried wearing contacts in 7th grade, but could never get used to them.
 * 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 used to get stuffed in his own cello case.
 * 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 is only main male character who wears glasses.
 * 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, Lisa and Bernadette Rostenkowski).
 * He is afraid of horses.
 * 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 always wears shorts when he goes to Kurt's apartment for Penny's errands.
 * Leonard is a quarter inch taller than Howard.
 * Leonard is always wearing a t-shirt when he is in bed with Penny.
 * Like Sheldon, Penny, Rajesh, and Amy, Leonard became a reverend to marry Howard and Bernadette.
 * 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 China.
 * As he stated in "The Contractual Obligation Implementation," when he was in middle school he wanted to be a rap star like Snoop Dog, but with "a healthy respect for the police." He states his parents pushed him into becoming a scientist.
 * 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.
 * 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 being nominated for Nobel Prize. Just hope Chuck Lorre and Bill Prady write that in sometimes, where Leonard gets nominated for Nobel Prize with Howard or Raj.
 * 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.