List of The Big Bang Theory characters

The following is a list of characters from the American situation comedy The Big Bang Theory created and executive produced by Chuck Lorre and Bill Prady, which premiered on CBS on September 24, 2007. It concerns two prodigies in their 20s, one a theoretical physicist and the other an experimental physicist, who work at Caltech and live across the hall from a waitress with show-biz aspirations. Their geekiness and intellect are contrasted by her social skills and common sense.

Regular

 * Dr. Leonard Leakey Hofstadter (Johnny Galecki) is an experimental physicist with an IQ of 173 who received his Ph.D. when he was 24 years old. He formerly shared an apartment with colleague and friend Sheldon Cooper. The writers have toyed with a romance between him and neighbor Penny, with their unresolved sexual tension being a major force for drama. Leonard dated Penny for most of Season 3, and while they have since broken up, some sexual tension is still apparent, but they rekindle their relationship in mid-Season 5. Previous to Penny, Leonard had romantic relationships with co-worker Dr. Leslie Winkle, physician Dr. Stephanie Barnett, an unnamed French literature PhD, and North Korean spy Joyce Kim. Following his breakup with Penny, Leonard has slept with Raj's sister Priya, and it is implied this also occurred on another occasion when Priya was visiting Raj (before Leonard met Penny). Leonard's family includes three other accomplished scientists: his mother, Dr. Beverly Hofstadter who is a psychiatrist and neuroscientist, his father Dr. Alfred Hofstadter who is an anthropologist and his sister (name unknown) who is a medical researcher, while his brother Michael Hofstadter is a Harvard law professor. Leonard wears glasses and is lactose intolerant. In the penultimate episode of Season 7, Penny and Leonard became engaged and were married in Las Vegas in the premiere episode of Season 9, followed with a re-wedding (while still married) in the premiere episode of Season 10. Later in season 9 they share living accommodations in both Penny's and Sheldon's apartments; But then in Season 10, Leonard and Penny live togther in Apartment 4A alone when Sheldon and Amy move in together into Apartment 4B, first as a living experiment, then permanently.
 * Dr. Sheldon Lee Cooper (Jim Parsons) is a theoretical physicist with an I.Q. of 187, possessing a B.S., a M.S., a M.A., a Ph.D., and a Sc.D. Originally from East Texas, he was a child prodigy, starting college at the age of 11, and receiving his first Ph.D. at age 16. He is usually cold, calculating, arrogant, selfish, immature, condescending and cynical, but also has a softer, loving and caring side as he does care about his friends and loves his mom, grandma and girlfriend Amy even though he can sometimes insult them with either about their intellect levels or different opinions and is also formerly asexual. Sheldon exhibits a strict adherence to routine, a lack of understanding of irony, sarcasm and humor, and a complete lack of humility; these characteristics are the main sources of his character's humor and the center of a number of episodes. Sheldon's family is very different from him; his father, George Sr., was a former store worker who died before the start of the series, while his mother, Mary, is a devoted Christian, and his twin sister, Missy, is a hostess at Fuddruckers and his older brother George Jr. is certainly no scientist. Sheldon is consistently the geekiest and quirkiest character in the show. Whenever he tells a joke, or plays a prank, he concludes with the word "Bazinga!" to signal that he just told a joke. Sheldon is friends with and then becomes the boyfriend of Amy Farrah Fowler until they break up in season 9, but reconcile later in the season and finally engage in coitus. In Season 10, Sheldon and Amy move in together for a five-week experiment to test out their compatibility. Afterwards, they decide to move in together permanently. In the Season 10 finale, he proposes to Amy, but her answer is currently unknown.
 * Penny Hofstadter (family name unrevealed) (Kaley Cuoco), (appeared in all but two episodes in season 4, but credited for the episodes in which she did not appear) is Leonard Hofstadter and Sheldon Cooper's neighbor across the hallway. Originally from Omaha, Nebraska, she is a waitress at the local Cheesecake Factory and also an aspiring actress. Penny is very outgoing, kind, and assertive, her personality contrasting with those of the guys. She has dated several men during the course of the series, including former boyfriend Kurt, Stuart (from the comic book store), and Zack. Penny toyed with the possibility of dating Leonard more than once throughout the show. They dated for most of season 3 but broke up; However, they got back together in mid-season 5. She has shown to be influenced by her geek friends, including an obsession with online gaming and making references to Star Trek, to her own surprise. She even remarked after a bad date with Zack that Leonard had ruined stupid guys for her. For the most part she cannot stand Sheldon's nerdy, eccentric personality, but at times they have shown to be a sister to him. Penny learns to love Leonard's good guy personality and eventually gets engaged to him in the penultimate episode of season 7 and marries him in the season 9 premiere, followed with a re-wedding (while still married) in the Season 10 premiere. Later in season 9 they share living accommodations in both Penny's and Sheldon's apartments, until Season 10 when Sheldon and Amy move into Apatment 4B, first as a living experiment, then permanently and Penny and Leonard live in Apartment 4A.
 * Howard Joel Wolowitz (Simon Helberg) is a Jewish engineer at Caltech's Department of Applied Physics who often hangs out at Leonard and Sheldon's apartment. Unlike Sheldon, Leonard, and Rajesh Ramayan Koothrappali, Howard lacks a doctorate. He defends this by pointing out that he has a master's degree in Engineering from MIT. Howard fancies himself a ladies' man and provides outrageous pick-up lines whenever a female is present. He still lives with his mother, who is overly oblivious to his accomplishments as an engineer. Howard is allergic to peanuts, and they are a serious threat to his life. As of season five, he has since married Penny's friend, Bernadette Rostenkowski and is now a father to a girl named Halley.
 * Dr. Rajesh Ramayan "Raj" Koothrappali (Kunal Nayyar) is Howard Wolowitz's best friend, and yet another genius of the group; his name is usually shortened to "Raj". He is originally from New Delhi, India, and he works in the Physics department at Caltech, where his area of expertise is particle astrophysics. Raj is extremely shy when it comes to dealing with women; he is unable to speak to them unless he drinks alcohol or is on medication. When Penny is around, Raj usually whispers what he wants to say to Howard or Leonard, who then responds out loud. Despite his pathology, Raj has often ended up in bed with women, leaving the other guys perplexed. Raj communicates with his parents, Dr. V.M. Koothrappali and Mrs. Koothrappali, in India through a webcam. He can finally talk to women without getting drunk in the season 6 finale and becomes the boyfriend of dermatologist and horror enthusiast Dr. Emily Sweeney in season 7. In season 9, Raj breaks up with her when he decides to pursue a cute screenwriter/bartender named Claire whom he met at the comic book store, but she turns him down, although she later calls him back to ask him out. He instead chooses to get back with Emily after she also calls him at the same time because she misses him and Raj now can't decide between Emily and Claire and had been seeing both of them at the same time, until Raj tells Stuart in "The Hot Tub Contamination" that he has broken up with both ladies, at their insistence. He then started seeing a Caltech cleaning lady, Issabella, but they broke up later.
 * Dr. Leslie Winkle (Sara Gilbert) (2.02-2.16, Guest in 9.17) is an experimental physicist who works in the same lab as Leonard. Her first appearance was in the third episode of season one. In appearance, she is essentially Leonard's female counterpart, equipped with the black framed glasses and sweat jackets. She also happens to have a lazy eye. She is one of Sheldon's arch-enemies, due to their conflicting scientific theories. Though they both consider each other to be intellectually inferior, Leslie is much wittier than Sheldon, regularly calling him "dumbass" and besting him in their exchanges. Leslie has had casual relationships with Leonard and later Howard, considering it a completely physical reaction to her body's cycles – her voice remains emotionless even during moments of extreme physical intimacy. Leslie's final appearance was in the season 3 finale "The Lunar Excitation", as the writers said they did not know how to work in the character full-time. After Penny broke up with Leonard, she got intoxicated, had intercourse with him, then pushed him away. Distraught, Leonard asked for intercourse with Leslie. Leslie responded, "Let me think about it," before slamming the door on him. However, she reappeared in the 17th episode of Season 9 where she was a guest at Sheldon's birthday party.
 * Dr. Bernadette Maryann Rostenkowski-Wolowitz (Melissa Rauch) (4.04-present) is a microbiologist that used to work at The Cheesecake Factory with Penny to pay for her grad school. Penny set her up with Howard and the two found out that they have the same issue with their overprotective mothers. This similarity brought the two together. They got in a relationship which ended when Bernadette found out that Howard was having cyber sex with Glacinda the troll. Not long after, they decided to rekindle the relationship, and the two got engaged during The Herb Garden Germination. Bernadette has a squeaky voice and when she is angry, she sounds exactly like Debbie, and Debbie approves of Bernadette, despite Bernadette being a Catholic. Unlike Howard, she dislikes children as she had a tough time raising her younger siblings, and she agrees to have kids with Howard only if he would stay at home with the kids instead. Bernadette has a father who is a retired police officer and is not very fond of Howard, a mother who drives her crazy and five siblings (Brothers and sisters with one of her brothers named Joey). She and Howard eventually got married on The Countdown Reflection, shortly before Howard left for space. She was worried that they won't move out of his mother's house, but they did, into Bernadette's apartment. However, since the passing of Howard's mother, they have moved back into her house and Bernadette gets pregnant. She later gives birth to a little girl named Halley.
 * Dr. Amy Farrah Fowler (Mayim Bialik) (4.08-present) is a neurobiologist who was determined to be Sheldon's perfect match by an online dating site that Howard and Raj secretly signed him up on. Amy is very plain, and the way she carries herself makes her the female version of Sheldon. Amy was Sheldon's "friend, who's a girl, but not his girlfriend" until they became official during "The Flaming Spittoon Acquisition". Amy became the latest member of the gang, and quickly became best friends with Penny and Bernadette, who taught her to be more like a normal girl and do girly stuffs with her. Amy admires Penny a lot and calls her her "Bestie", Not much about Amy's family is known, but her mother has appeared once before through a video chat session with Amy, a great-aunt Flora who is 93 years old and two cousins that died. Amy's family has been described to be very nosy about her romantic life. She is Sheldon's girlfriend from season 5 to season 9 when she gets frustrated with him and they break up. By the tenth episode of the season, Shamy gets back together, are more passionate than ever and have sex in the eleventh episode of the season. She also learns that Sheldon had planned to propose at the time of their break up. In Season 10, after a plumbing accident makes her apartment uninhabitable, Sheldon and Amy embark on a five-week experiment living together to test out their compatibility. Amy later admits she lied about how long the repairs were taking so that she could continue living with Sheldon. They eventually move into Penny's old apartment together, while Penny and Leonard live alone in Apartment 4A. Afterwards, they decide to move in together permanently. In the Season 10 finale, Sheldon proposes to her, but her answer is currently unknown.
 * Stuart David Bloom (Kevin Sussman) (6.01-6.13, 8.01-present) runs the comic book store that the guys frequently go to. He is also a nerd, but he has a talent for drawing, is a graduate of Rhode Island School of Design and possesses a few more social skills than they do. During Stuart's first appearance, the guys brought Penny along to the store and he managed to ask her on a date. The date went fine until Stuart was dragged into discussing comic books by Sheldon. On a second date with Penny, Stuart asked for Leonard's advice, but feeling threatened, Leonard ultimately gave him bad advice. The date went fine, but when Penny was getting romantic with Stuart she accidentally called him "Leonard," leaving him devastated. Stuart was the partner of Wil Wheaton in the card game tournament, where they defeated the pair formed by Raj and Sheldon. As Stuart runs a comic book store, he has an almost encyclopedic knowledge of comic books and superheroes. In a recent episode, he implied he was in financial trouble and that the comic book store is now also his home. Stuart was also at one point, attracted to Amy when she treated him nicely and managed to get her to go on dates with him. He became the reason for Sheldon and Amy to become official when a jealous Sheldon finally asked Amy to be his girlfriend after he realized he does not like the idea of Amy going out with other men. Stuart moves in Debbie Wolowitz's house to care for her after she breaks her leg and his comic book store is gutted in a fire. Then he never seems to leave even after Howard and Bernadette move in, but does so later in the twelfth episode of Season 9, before moving back in in Season 10.
 * Dr. Emily Sweeney (Laura Spencer) (9.04-9.18) is a dermatologist who Raj begins dating after he finds her on a dating site. She has something against Penny because she had fooled around with Raj at the end of Season 4. She is a fan of horror movies. Valentine's Day 2016, Raj breaks up with her when he thinks they'e not right together and because he met a cute screen writer named Claire whom he felt interested in. Later, Emily calls Raj and tells him she misses him and he goes to her apartment and they hook up. At that moment, he was seeing both Emily and Claire, until they both broke up with him at the girls' insistence. She is now in a happier relationship with a man called Gary which is revealed when Raj and Howard invite Emily, Claire, Lucy and the deaf Emily to Raj's flat for an intervention to find out why women keep dumping Raj. Emily mentions that though she enjoyed her sexual relationship with Raj, she was put off by his fear of his parents. She says that he was also bad in bed and believed with the def Emily that he is such a momma's boy.

Recurring characters
These characters appear in two or more than two episodes. The list is sorted by chronological order of appearance, considering their first appearance in the show.
 * Althea (Vernee Watson): A character that made an appearance in the Pilot as an attendant at a sperm bank, and then made two appearances as a emergency room nurse in The Peanut Reaction episode (season 1), and The Robotic Manipulation episode (Season 4) and one other appearance as a ward nurse in The Werewolf Transformation episode (Season 5). The name of the character is never used on the show, only on her name tag and in the credits.
 * Kurt (Brian Patrick Wade): A tall, muscular intimidating bully, Kurt is Penny's ex-boyfriend at the beginning of the series. In the pilot episode Leonard and Sheldon went to his apartment and tried to retrieve Penny's TV set from him, but they did not succeed and returned home without pants. Penny left him because he cheated on her, but he was still invited to her Halloween party. When Penny experienced financial difficulties, Leonard and the guys tried to convince Kurt to pay Penny back money he owed her; at first Kurt refused, but eventually he paid Penny back to date her again.
 * Dr. Eric Gablehauser (Mark Harelik): The head of the Physics Department, Dr. Gablehauser is the guys' boss at the university. In his first appearance, he fired Sheldon after Sheldon insulted his intelligence. Eventually, Sheldon was rehired because Gablehauser became romantically interested in Mary Cooper, Sheldon's mother. Gablehauser referred to Sheldon, Leonard and Rajesh as doctors, but to Howard as "mister" because he lacks a Ph.D (but carries a Masters Degree). Dr. Gablehauser was also responsible for introducing child genius Dennis Kim to the university and for hosting the Physics Bowl.
 * Mary Cooper (Laurie Metcalf): Sheldon's loving and caring mother and a devout Christian from Texas. She is a church worker and also has two other children besides Sheldon; Sheldon's fraternal twin sister, Missy and his brother George Jr., who is shown via a family tree in "The Cruciferous Vegetable Amplification" to be three or four years older than Sheldon and Missy (depending on birth month and year). She first appeared in the fourth episode of the first season and had also appeared in the season three premiere, the third episode of season four, the sixth episode of season 5, the eighteenth episode of season 7, the pre-ultimate episode of season eight, the season nine premiere and the season nine finale, and was heard in the eleventh episode of season 7. Mary herself is not intellectual but is very wise; she appears to be an extremely good mother and is the only one who has ever been able to control Sheldon, although Bernadette can as well. She twice came to Pasadena when Sheldon had lost it and once when she was taking a cruise out of LA. Sheldon has also run back to Texas where his mother convinced him to return and once to surprise her and Sheldon could her having sex. Leonard described Mary as Sheldon's "Kryptonite". When Penny and Sheldon engaged in a fierce dispute, Penny called for Mary's help, who subsequently called and scolded Sheldon about his actions. Mary meets Beverly Hofstadter and their personalities clash, once in season 8 and once again in season 9. She also meets Leonard's father Alfred Hofstadter in the season 9 finale where Sheldon invites her to Leonard and Penny's re-wedding and the two of them bond over their mutual dislike of Beverly and their interest in each other's beliefs and they get along great during dinner and it is implied that the two hook up, until the next episode (The season 10 premiere: "The Conjugal Conjecture") where it is revealed that they didn't sleep together, but plan on continuing to see each other in the future.
 * Chen (James Hong): The owner of a Chinese restaurant the guys frequently visit.
 * Debbie Melvina Wolowitz (voiced by Carol Ann Susi): Howard's overbearing, over-protective, controlling, belittling, but loving and caring stereotypical Jewish mother, she is never seen on-screen but her voice is heard when he is at their house or when he talks to her on the phone. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. Debbie seems to be oblivious about Howard's work as an engineer, and talks to him like he is still a child. She frequently refers to Leonard, Sheldon and Raj as Howard's "little friends" when yelling to him (as in, "I made some cookies, do you and your little friends want some?") She makes Howard's life miserable at home, which prompts him to call her a "crazy old lady." However, in one episode, Howard attempted to move out, but had serious delusions that his mother was trying to force him to stay, when in actuality she was urging him to move out, so he stayed, but despite problems, Howard is deeply attached to his mother and quite happy to be treated like a child. However, in one episode, Howard attempts to move out after a fight with his mother, but has serious delusions that she is trying to force him to stay, when in actuality, she is urging him to move out, which leads to him staying. It is also mentioned that she is on hormone replacement therapy and she wants to "play doctor" with Raj when he asks to sleep over. In the Season 7 finale (The Status Quo Combustion) Stuart called Mrs. Wolowitz "Debbie" so this is her real name. In Season 8, she died, taking a nap and never waking up afterwards. After her death, Howard and Bernadette move back in and Stuart continues,living with them until he moves out in Season 9's "The Sales Call Sublimation", but moves back in during Season 10.
 * Dr. V.M. Koothrappali (Brian George) and Mrs. Koothrappali (Alice Amter): Raj's parents in India, they communicate with their son via video chat and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent, and give them grandchildren. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is "that Jewish boy", Howard. Although in many episodes Raj mentions that he grew up in poverty, his friends remind him of the contrary by pointing out that Dr. Koothrappali is a gynecologist and drives a Bentley. The Koothrappalis enjoy Doogie Howser reruns, which are apparently new to India. In season 8, they were divorced. Raj's father is currently the only parent to visit Pasadena. In season 10, Raj called him to give advice to a pregnant Bernadette.
 * Missy Cooper (Courtney Henggeler): Missy is Sheldon's fraternal twin sister who is intellectually and socially very much unlike him. Confident, tall and attractive, she immediately catches the attention of Leonard, Howard, and Raj. Sheldon realizes that within Missy's eggs lies the potential for another "superior mutation" like him. He temporarily erects himself as a guardian to Missy in order to choose the best mate for her. Missy refers to Sheldon as "Shelly", and despite their differences she loves him and is proud of his accomplishments. In "The Cooper Extraction", she gives birth to a baby boy with Sheldon helping while her husband is recovering from a motorcycle accident. She is only heard screaming once.
 * Dr. Ramona Nowitzki (Riki Lindhome): A female graduate student who is a huge fan of Sheldon's work and becomes Sheldon's first groupie and Sheldon exploits to get free food. She meets Sheldon after he introduced his work to her and other incoming graduate students. She arranges to have dinner with him in his apartment. The next few days she becomes a permanent presence in his life, pressuring him to concentrate on his research and avoid distractions. Sheldon gets tired of her but is unable to dissolve their "relationship". Eventually, Sheldon reaches a breakthrough in his work, and kicks Ramona out when he refuses to share credit with her for the discovery. She returns in "The Long Distance Dissonance" where she now works at Caltech and has a Ph.D. and her huge crush on Sheldon happens again. The gang is worried and keep their eyes on Ramona. When she kisses him, Sheldon flies out to New Jersey to propose to Amy.
 * Dr. Stephanie Barnett (Sara Rue): A doctor and highly distinguished surgical resident at Fremont Memorial, Stephanie is first seen as Howard's date, but later becomes Leonard's girlfriend. When Howard's plan to let her drive the Mars rover failed, she left with Leonard and the two immediately became attracted to each other. Sheldon began meddling in Stephanie and Leonard's new relationship in an effort to consolidate it, since in his eyes, Stephanie was the only "tolerable" mate Leonard had had. After several weeks of dating, Stephanie officially began living with Leonard in his and Sheldon's apartment. However, after much hesitance, Leonard told her to move out because he was uncomfortable with the pace of the relationship. Although they weren't seen breaking up on camera, evidence suggests their relationship ended. Series co-creator Bill Prady confirmed this in an interview, stating: "Stephanie was a chance for Leonard to learn that just because someone loves you, doesn't mean you'll love them back". Stephanie did her medical internship at Lawrence Memorial in Galveston, Texas, which is where Sheldon was born.
 * Barry Kripke (John Ross Bowie): An unlikable co-worker of Leonard and Sheldon's, who works in plasma physics, Kripke has a case of rhotacism where he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes. In his first appearance, he pitted his robot, the Kripke Krippler (or, as he called it, the "Kwipke Kwippwer"), against the guys' robot, M.O.N.T.E., in an unofficial robot fight. On another episode, Sheldon attempted to befriend Kripke in order to gain access to an open science grid computer to carry out research, but it turned out to be futile, as Kripke had no control of the computer as Sheldon had thought. Kripke continued his antagonism towards Sheldon, when he pulled a prank on Sheldon when the latter was a guest on NPR's Science Friday. Sheldon tried to befriend Kripke another time when he was feeling left out from his friends, but Barry connected more with the others in Sheldon's new proposed group. In season 9, Barry began teaching the guys how to fence, calls Sheldon and Amy's live show Fun with Flags along with Raj and directs the conversation to irrelevant questions to the show. He also uses the show to call out for potential romantic relationships and asked Amy questions about her love life when Sheldon and she broke up. He attends Sheldon's birthday party and constantly hits on Leonard's mother, Beverly.
 * Dr. Beverly Hofstadter (Christine Baranski): Leonard's narcissistic, unloving and overly analytical mother. Beverly is a neuroscientist as well as a psychiatrist. The only reason she had children was to use them as personal experiments for her research, and thus was very distant and unmotherly and also a bad wife to Leonard's father. Between her neurotically strict speech patterns, lack of social conventions, and attention to detail, she is Sheldon's female equivalent. The two of them share an odd non-romantic attraction to each other, which culminates in her kissing him after becoming drunk. Though still married to Leonard's father, she revealed in her second appearance that the two were getting a divorce. She had frequently implied that the two haven't had "coitus" since Leonard was conceived. She diagnosed Raj with selective mutism and considered Raj and Howard's relationship to be an "ersatz homosexual marriage". Beverly mentioned that Leonard's brother and sister are more successful in their respective fields than he is. Ironically, as displayed in "The Maternal Congruence" episode, Sheldon is closer to Beverly than her own son is. In season 8, she visited to see Leonard get an award and got into a disagreement with Mary Cooper over her religious beliefs who in return berated Beverly for her cold and distant attitude towards Leonard. In season 9, she attended Sheldon's birthday party and enjoyed getting hit on by Barry Kripke. Later, she reveals that she's insulted about not being invited to her son's wedding and then Penny suggests a re-wedding which Beverly accepts and during the pre-wedding dinner, ends up fighting with her ex-husband and once again with Mary Cooper. During the wedding ceremony, she agrees with her ex-husband that Leonard was something wonderful to come out of their relationship. In both 2009 and 2010, Christine Baranski was nominated for an Emmy Award for Outstanding Guest Actress in a Comedy Series for this role.
 * Wil Wheaton (portraying a former antagonistic version of himself): In the season 3 episode "The Creepy Candy Coating Corollary", Sheldon enters a card game tournament (Mystic Warlords of Ka'a) to confront Wil Wheaton over an incident that occurred in 1995 when Sheldon was devastated because Wheaton (Wesley Crusher in Star Trek: The Next Generation) failed to show up at a fan convention that Sheldon attended. In the final match, Sheldon is about to defeat his hated rival, but Wheaton lies to him about the reason for missing the convention, and Sheldon throws the game out of guilt. Wil Wheaton makes a second appearance in the episode "The Wheaton Recurrence", where he breaks up Leonard and Penny's relationship in order to win a bowling competition against the main characters. In "The 21-Second Excitation", Wheaton appears at a screening of Raiders of the Lost Ark, in which he uses his celebrity status to go to the front of a long line. This antagonizes Sheldon, as Wheaton and his entourage effectively prevent Sheldon and the guys from attending the screening when the last seat in the theater goes to the person directly in front of them in line. Ultimately, Sheldon steals the prints of the film and Wheaton leads a mob of angry Raiders fans after Sheldon, Howard, Raj and Leonard. In later seasons, Wil apparently has a change of heart in his attitude towards Sheldon when he brings him [Sheldon] his [Wil's] last remaining action figure of himself, mint and signed, to make up for the convention years ago where he was a no-show. Afterwards, Sheldon and Wil become friends, Wil appearing on Sheldon's Fun With Flags web-show and consoling Sheldon over his blunder with a putative new chemical element. Sheldon also has a spat with Wil when the latter has a disagreement with Amy and Sheldon fails to take Amy's side (for want of understanding why he should). A drunk Sheldon then shows up at Wil's house at night to challenge Wil to fisticuffs, but ends up puking in Wil's bushes. Wil seems to not take the whole thing personally. In later seasons, Wil makes a movie (Serial Apeist 2) with Penny, interviews her on his podcast about the movie, interviews Sheldon for a documentary on Mr. Spock with his son Adam Nimoy and attends Sheldon's birthday party.
 * President Siebert (Joshua Malina): The president of the California Institute of Technology; he is married and has a teenage daughter. Siebert sends Sheldon on an expedition to the North Pole at the end of Season 2. In Season 4, he organizes a get-together with wealthy donors and faculty in order to garner funds for the university's programs. He urges the guys to make an appearance to schmooze the donors so that they will give to their projects. Seeing Sheldon's behavior toward the donors, he asks that he not appear again.
 * Zack Johnson (Brian Thomas Smith): A dim-witted, but friendly beefcake-type whom Penny dates after her break-up from Leonard. Leonard invites Penny and him up on the roof to watch their experiment bouncing laser beams off the moon. Zack has no clue what is happening. Later, he and Penny go out on a date; she is so put off at how stupid he is that she runs back to Leonard to have sex, saying that Leonard has ruined stupid guys for her. In a later episode, he runs into Penny and Amy at a restaurant, and Amy becomes physically attracted to him. Unique among Penny's boyfriends, he considers Leonard, Howard, Sheldon and Raj (whom he calls "the science guys") to be "cool" and makes a genuine effort to get to know them. After accompanying them on a trip to the comic book store (he and Raj bond over a mutual fondness for Archie comics), he enters a New Year's Eve costume party with them, portraying Superman in their version of the Justice League. He apparently considers them his friends. He works as the menu designer for restaurants either owned or who employ his father's company to design their menus. More recently, he seems to have befriended Kripke and Stuart as well. Penny and Zack got married in a chapel in Vegas one Thanksgiving and thought the weddings were fake, but they later annulled the marriage in "The Thanksgiving Decoupling" three years later and Zack showed up in Leonard's story on what would've happened to Penny if she and Leonard never met.
 * Priya Koothrappali (Aarti Mann): Raj's younger sister. In "The Irish Pub Formulation," it is implied that she and Leonard hooked up sometime earlier when she was in town and that Leonard had been much more invested in the relationship than she. Leonard appears to have strong feelings for her, even offering to relocate to Delhi to be with her. He tries to keep their relationship a secret from the guys, but Sheldon discovers Priya in their apartment. Leonard eventually reveals to the rest of the guys that he had spent the night with Priya, as he is unable to bear the burden of keeping the secret. Priya returns to Los Angeles for business in "The Cohabitation Formulation," and she and Leonard decide to try and have a real relationship, over the protests of Raj. Priya soon becomes jealous of Penny, and asks Leonard to stop spending time with her. Priya contests Sheldon's roommate agreement with Leonard, employing her Cambridge legal background to deny Sheldon power over Leonard, but Sheldon drafts a new agreement and blackmails Leonard and Priya into Leonard signing it. Priya then moves back to India, but her relationship with Leonard continues via physical distance and breakfast/dinner dates via Skype. Leonard later tells Priya of having had a one-time kiss with a girl called Alice and when Priya forgives him, she admits to Leonard that she cheated on him by sleeping with her ex-boyfriend a couple of weeks ago, and as a result, their relationship ends. It is revealed by her father in "The Allowance Evaporation", that she is married and self-sufficient, like her sister and three other brothers.
 * Wyatt (Keith Carradine): Penny's father who first appears in the season 4 episode "The Boyfriend Complexity when he comes to visit and Penny feels compelled to involve Leonard in a ruse to make him think that they are still together. (Wyatt reveals that he favors the successful Leonard over Penny's other "loser" boyfriends.) When the plot is revealed, he gets mad at Penny and feigns anger at Leonard. But after Penny leaves the room, he encourages Leonard to keep trying to get Penny back (Because he wants his grandchildren to "grow up in a house without wheels"). Strangely though, in the first episode where Leonard's mother visits, Penny tearfully reveals her rough childhood, saying that Wyatt wanted a boy and even called her "slugger" until she reached puberty. Wyatt appears again in season 9, when Penny finally plucks up the courage to tell her father she eloped with Leonard. Wyatt is happy, but he also confesses that he killed Penny's pet pig a year ago. In the season 10 premiere, "The Conjugal Conjecture", he comes to Pasadena with his wife and son to attend Leonard and Penny's re-wedding and during the ceremony he walks Penny down the aisle, welcomes Leonard to their family and advises him not to lend Penny's brother any money.
 * LeVar Burton (Himself): In the season 4 episode, "The Toast Derivation", Sheldon invited Burton to a party via a tweet, but doesn't make it quick enough. But Sheldon leaves after he feels his guests were having fun the wrong way and misses LeVar's presence when he arrives at the apartment moments later. Upon opening the door and seeing the other guests at the party (Zack, Kripke, and Stuart {who at that moment was wearing nothing but a towel}) singing karaoke, Burton leaves in disgust, swearing he'll never follow Twitter again. In "The Habitation Configuration" (S06E07), LeVar was the guest star for Sheldon and Amy's "Fun with Flags" episode about Star Trek flags. Sheldon commented that it surprisingly only took gas, free food and money to get him there. After Amy objected to his acting as episode director, stating he is worse than Wil Wheaton, Sheldon explained that he had to take his girlfriend's side. LeVar told him that he totally understood and questioned if he still got lunch. Then, a timid Sheldon nodded, so LeVar seemed happy, while Amy shook her head in frustration. In "The Champagne Reflection", LeVar was again the guest star for Sheldon on a "Sheldon Cooper presents Fun with Flags" episode because Sheldon was willing to delete Levar's contact information from his computer which Levar would witness. On the show, Sheldon lets him view a clip of his George Washington Carver impersonation (for Black History month) that Leonard thought was "wildly racist". Levar is also shocked though Sheldon thinks that he is agreeing with his criticism of Leonard. After Sheldon restarts the "Fun with Flags" series, he goes to Levar's house and asks him to guest star on an episode about German flags since he was born in Germany to American parents. He asks Levar to dress up like a swastika and gets the door slammed in his face. Levar also says that he should have listened to Will Wheaton and gotten a gate.
 * Emily (Katie Leclerc): A deaf girl Penny knows from her spin class and who dates Raj only for his money and they break up when he says they have to return all the gifts he got her after his parents threaten to cut off his allowance if he doesn't stop seeing her. She reappears in "The Emotion Detection Automation" where it is revealed that the reason she broke up with Raj was because she considered him a momma's boy and he was so dominated by his parents.
 * Dale (Josh Brener):A guy who works as a replacement for Stuart at the comic-book store when he goes on a date with Amy in "The Flaming Spittoon Acquisition", and also appears in "The Tangible Affection Proof" at Stuart's Valentine's Day party.
 * Mike Rostenkowski (Casey Sander): Bernadette's father and a tough, retired police officer. Mike has a rough and rude personality and often bullies others, although he deeply loves his daughter, calling Bernadette his "little girl". Before he is first seen, Bernadette establishes his personality by telling Howard a long list of subjects he cannot discuss with Mike, including Jimmy Carter, foreigners, homosexuals, and even Howard's Jewish identity. In his first appearance, he tells Howard that, while he does not think highly of him, his respect for Howard grew when he learned he had been selected to become an astronaut. However, after Howard expresses his fears about not returning from space, Mike believes Bernadette will have no trouble finding another suitable bachelor if Howard dies in space, much to the future astronaut's dismay. In the season 5 finale, Mike attends Howard and Bernadette's wedding and walks his daughter down the aisle. In season 6, Howard and Mike are pushed into a fishing weekend by their spouses, but neither of them really wants to go. Mike suggests that they go to a casino to shoot craps. Having earned more of Mike's respect, Howard is now able to address him as "Mike" rather than "Sir". In season 7, he comes to Howard's mother's house for Thanksgiving dinner where he watches football. Since Sheldon understands football from growing up with his father, Mike and Sheldon bond and drink heavily together. After insulting Howard with Sheldon and getting yelled at by Bernadette, Mike later tells Howard that he isn't "all that bad" but implies that this statement is untrue when he comments on his drunken state. In "The Table Polarization", Mike, Bernadette, Raj, and Mike Massimino (via Skype) perform an intervention on Howard's decision to go back to space. Mike mentions that Howard asked him to shoot him in the foot to avoid the first space mission, which Howard insists was a joke. In season 9, he comes to help redecorate Howard and Bernadette's house and wonders why Howard doesn't want kids, but Howard tells him that it's Bernadette who doesn't want kids and later, it is revealed that his wife did everything of raising the kids while all he did was come home from work, sit on the couch and drink beer.
 * Mrs. Rostenkowski (Meagen Fay): Bernadette's mother who first appears in "The Countdown Reflection" in a cameo from above her daughter's wedding with her face unseen and then she appears again, this time physically and with her face shown in "The Fish Guts Displacement" where she and her husband have dinner with Howard and Bernadette. She is a bit scatterbrained, but is able to get her husband to do things and usually makes Bernadette crazy like Howard's mother does to him. The fact that they both have overbearing mothers is how Howard and Bernadette bonded on their first date and got together.
 * Alex Jensen (Margo Harshman): An attractive Caltech graduate student who appears in season six. She is hired by Sheldon to review his kindergarten and elementary school notebooks for any possible Nobel Prize winning research and attend to the tasks that Sheldon perceives a waste of his own valuable time. Amy is at first jealous until she sees Alex instead flirting with an unresponsive Leonard. After repeated flirts by Alex, Leonard finally realizes that Alex is making advances toward him. While proclaiming his loyalty to Penny, Leonard is flattered. Sheldon interferes (with the perceived intention of mitigating a situation that needed none) by lecturing Alex, but only succeeds in insulting her. After Alex files a sexual harassment complaint, Sheldon is forced to apologize and is ordered take a university-mandated online sexual harassment course. Because Sheldon considers his own time is too valuable to waste on it, Sheldon asks Alex to take the course instead. Alex later scores some points with Sheldon in "The Tangible Affection Proof" after he sends her out to buy Amy a Valentine's Day gift, and Sheldon likes it so much that he keeps it for himself.
 * Janine Davis (Regina King): A human resources representative working at the university. In season 6, after Sheldon offends his assistant Alex whilst trying to solve a problem regarding her crush on Leonard, Janine is left to handle a sexual harassment complaint against Sheldon by Alex. Sheldon tells Janine she is a "slave" to her "sexual urges" and in his attempt at defense, rats out "worse behavior" by Raj, Howard, and Leonard, prompting Mrs. Davis to summon them to her office as well. Janine later serves on the tenure committee that is considering Raj, Sheldon, Leonard and Kripke for a vacant tenured position, causing each of them to try to "butter up" Mrs. Davis. Sheldon, in an attempt to win her over, yet again offends her by giving her the boxset for the TV series "Roots", and when she asks Sheldon why he thought this would be an appropriate gift Sheldon asks "you are black, right?". Despite this, Sheldon is still short-listed for Tenure, however he offends Janine yet again by giving her what he considers a traditional "black" handshake. In season 7, Janine is revealed to be recently divorced after her husband cheats on her, and Raj attempts to strike up a conversation with her at a university mixer. After offending her, Raj apologizes and the two talk about their troubles. While Raj is insistent that the two of them had a "moment", Howard disagrees.
 * Lucy (Kate Micucci): A woman who shows up at a party at the comic book store on Valentine's Day for people who had no dates. She has social anxiety issues, which allows her and Raj to connect. On their first date, she becomes so nervous that she goes to the bathroom and climbs out of the window to get away. Raj falls apart and refuses to leave his apartment; however after apologizing to him, Lucy and Raj start afresh. Raj and Lucy have a date at a library and communicate via text messages and, at the end, she offers to kiss him, but chickens out at the last second. In "The Bon Voyage Reaction", Raj tries to introduce Lucy to the rest of the gang. Lucy becomes nervous at the thought of meeting so many new people that Raj suggests she meet just one of his friends first, to which she agrees. Lucy meets Amy, however Raj pressures Lucy and upsets her. Lucy breaks up with Raj via an email, and afterwards a heartbroken Raj finally speaks to Penny without having consumed alcohol. In season 7, Penny meets Lucy at The Cheesecake Factory and confronts her about her breakup with Raj, and later Lucy arranges to meet with Raj to apologize to him for breaking up with him via email. This leads Raj to believe the two of them will get back together, however Lucy tells him she is with someone else. Lucy is later mentioned in "The Indecision Amalgamation". Raj gets a message from her about getting back together; however after finding out that he is also dating Emily, Lucy rebuffs Raj. She reappears in Season 10 to talk about why she broke up with him and she said he is too pushy. She also revealed that she is now in a better relationship.
 * Bert Kibbler (Brian Posehn): A socially awkward geologist who first appears in "The Contractual Obligation Implementation", when he meets Raj and Lucy in the library during their date. Later in "The Occupation Recalibration", he is found having a crush on Amy and tries to ask her out on a date to a geology rock show. In "The Dependence Transcendence", Bert hosts a Caltech party that only Amy and Penny attend. He feels that Amy is the coolest girl at work and that Sheldon is cool only because he is dating Amy. As the girls are leaving, he tells them that he is in love with both of them. He reappeared in "The Fetal Kick Catalyst" as a guest at Shamy's brunch in their apartment. He mentions that a girlfriend he was living with once moved out and took all his best rocks. He reappeared in "The Geology Elevation" after winning a $500,000 MacArthur Fellowship grant from Caltech that makes Sheldon jealous. He admits that he has a thing for Amy, but once he won his grant he thinks that he could do better. He also thinks about nominating Howard for another one which even more irritates Sheldon. He and Sheldon later make up and they go together to the Ellen Degeneres show. He reappears again in "The Separation Agitation" where he has a new girlfriend Rebecca, who only seems to be with Bert because of his money. They break up in the episode, but it is then revealed in the end that they are back together.
 * Arthur Jeffries, aka Professor Proton (Bob Newhart): The star of a science show that Sheldon and Leonard watched as children. After the show was cancelled, Jeffries was not taken seriously as a scientist and resorted to doing children's parties as his persona. In "The Proton Resurgence", Sheldon hires him to do a private party for Leonard and himself (and Penny). Later Jeffries ends up having a heart attack as a result of climbing the apartment stairs with his equipment. He asks Sheldon to take his place at a party for a Korean family, which Sheldon agrees to do as Professor Proton, Jr. While Sheldon idolizes him, Jeffries is constantly irritated by Sheldon's personality, and takes a shine to Penny. In "The Proton Displacement", Leonard, Sheldon and Amy bump into Jeffries at the drugstore, and Sheldon becomes upset when Jeffries asks Leonard for help on a paper instead of him, and when he finds out that Jeffries thinks he is annoying, Sheldon works with Bill Nye the Science Guy (he laters abandons Sheldon and gets a restraining order against him), whom Jeffries accuses of stealing material from his TV show. After a chat with Leonard, Jeffries later asks Sheldon for his advice on his paper (although Sheldon already hacked his email account and read it for himself). In "The Proton Transmogrification" on Star Wars Day, Leonard tells Sheldon that Arthur has died. Sheldon immediately pretends like he doesn't care and refuses to go to the funeral. Arthur appears to Sheldon in a dream, where Sheldon compares him to Obi-Wan Kenobi, dressing him in Jedi robes and making him appear as a Force ghost. When Sheldon asks for his wisdom, Arthur tells Sheldon to appreciate everything in life, including his friends, and never take it for granted. In "The Opening Night Excitation", he returned and advised Sheldon to see the new Star Wars movie another time (even though it is the premiere) and be with Amy on her birthday.
 * Dan (Stephen Root): Bernadette's and later also Penny's boss at Zangen. Bernadette arranges an interview for Penny with Dan. The interview goes terribly, but Dan eventually hires Penny when they bond over their shared fear of Bernadette. In "The Champagne Reflection", Penny has Bernadette face up to her bullying ways. Dan is still afraid of her and she yelled at him and his grandson after they lost the three-legged race at the company picnic. His grandson called her the mean kid with the big boobies. Bernadette is horrified when she realizes how everyone perceives her. Her father had taught her to not take any crap, and she can be even cruel with some of her comments without even knowing it. She thought she was the sweetest person she knew and should be in a tree baking cookies like a Keibler elf. She goes to apologize to Dan and finds out that due to his fear of her, they were paying for her daily coffee even after the company stopped doing that and gave her a private toilet on her floor that was meant for everyone. She now feels like she has to leave the company and starts crying. Dan wants her to stop and promises to keep paying for her coffee and then ups it to getting her an espresso machine. Bernadette has found a new way to get what she wants.
 * David "Dave" Gibbs (Stephen Merchant): A tall British man who dates Amy after she breaks up with Sheldon. Dave is recently divorced because his wife cheated on him and left him for a French chef. When her friends try to find Amy a date on a dating Mobile app, they see a text message from Dave, and Amy reveals they have already gone out together for coffee. Sheldon is about to propose to Amy for closure when he sees Amy and Dave share a kiss after their date. When Amy and Dave later go out again for dinner (while Penny, Bernadette, and a reluctant Leonard spy on them), he shows an unusual fascination with Sheldon, which discomforts Amy. He considers Sheldon and Leonard to be physics superstars and idolizes them. He later gets an opportunity to meet Leonard and shake his hand after Leonard has to relieve himself whilst spying on their date, and with great excitement, Dave announces that he will never again wash his hand. After Sheldon rejects Amy's proposal that they be a couple again, Amy once again tries dinner with Dave, who goes out of his way to refrain from mentioning Sheldon, knowing how uncomfortable that subject made her previously. Sheldon then suddenly arrives and professes his love to Amy, and the two mend their relationship, leaving Dave to awkwardly excuse himself and invite Sheldon to meet again on his way out.
 * Claire (Alessandra Torresani): a bartender and writer for a children's science fiction series who Raj and Howard meet in the comic book store. Claire wants to collaborate with Raj on the science portions of a movie script. She is only interested in his help while Raj has visions of marriage and children. After Raj breaks up with Emily, he calls Claire on Valentine's Day; however, she just got back together with her boyfriend. She also calls Raj an 'ass' for breaking up with Emily just before Valentine's Day, although later she asks Raj out for a drink when she again breaks up with her boyfriend. He is then forced to choose between Emily and her and he picks Emily, but it's later revealed that he is now seeing both of them at the same time. She reappears in "The Fermentation Bifurcation", where she goes to a wine tasting with Raj and some of the gang. Claire doesn't seem happy about the fact that Raj is seeing two women at once and at the wine tasting, she almost gets hit on by Penny's ex-boyfriend Zack whom Raj stops when he tells him that Claire is with him. After breaking up with Raj, Claire accepts an invitation from him to come to his apartment where she meets Raj's previous girlfriends and reveals that she is currently in a better relationship. She also reveals that she broke up with Raj because she felt he was too needy.
 * Dr. Alfred Hofstadter (Judd Hirsch): Leonard's father who is mentioned several times in the series before first appearing in the Season 9 finale "The Convergence-Convergence" where he is invited to Leonard and Penny's re-wedding. He is an anthropologist who once worked with famed British archaeologist Louis Leakey. During his visit for Leonard and Penny's re-wedding, he meets Mary Cooper and the two get along great, bonding over their mutual dislike of Beverly and their interests in each other's beliefs and the two end up fighting with her at the restaurant during the pre-ceremony dinner and the two leave together and after finding out they are staying at the same hotel, they decide to share a cab and it is implied that the two hook up, until the next episode (The season 10 premiere: "The Conjugal Conjecture") where it is revealed that they didn't sleep together, but plan on continuing to see each other in the future.
 * Colonel Richard Williams (Dean Norris): An Air Force Officer and Engineer who becomes interested in the guys' invention.
 * Halley Wolowitz (Pamela Adlon): Howard and Bernadette's baby daughter who is born in "The Birthday Synchronicity". She is only heard crying and her crying sounds like the way Howard's mother used to shout.

Minor characters
These characters appear in just one episode or in one episode for now until they make more appearances. The list is sorted by chronological order of appearance, considering their first appearance in the show.
 * Christy Vanderbel (Brooke D'Orsay): A friend of Penny from Nebraska, Christy suddenly decided to move in with Penny. Being promiscuous, Christy ends up sleeping with Howard, who in turn invites her to move in with him (and his mother). Debbie and Christy end up having a fight, and the latter leaves.
 * Lalita Gupta (Sarayu Rao): A childhood acquaintance of Raj, she is now a dental student at USC. Raj's parents set up a date for their son with her. During the date, Raj can only speak to her after drinking an alcoholic beverage (a "grasshopper"). Sheldon insists she bears a remarkable resemblance to Princess Panchali, an Indian princess in a children's story. Eventually, Lalita leaves the obnoxious and drunk Raj to have dinner with Sheldon.
 * Toby Loobenfeld (DJ Qualls): A research assistant with a double major in Physics and Theater, Toby was used by Sheldon to play his fictional first cousin "Leopold Houston" from Denton, Texas, a character Sheldon invents as part of an elaborate lie told to avoid going to see Penny sing. Cousin "Leo" is a recovering drug addict who ends up cuddling with Penny on the couch, much to the chagrin of Leonard.
 * Dennis Kim (Austin Lee): Dennis is a 15 year old child prodigy from North Korea whom Dr. Gablehauser hopes to recruit into their Ph.D. program. Dennis and Sheldon have an antagonistic relationship when the former excels Sheldon in every way. The guys make arrangements so Dennis can meet girls of his age and get distracted from his own research. The plan succeeds and Dennis is seen kissing with a girl, no longer an intellectual threat to Sheldon.
 * Dimitri (Adam Gregor): A Russian man who is the third floor janitor of the University's Physics Department and a former physicist in the former Soviet Union. He appears in "The Bat Jar Conjecture" where Sheldon recruits him, as well as the lady from the lunchroom and her son or butcher as part of his Physics Bowl team 'AA (Army Ants)'. Dimitri answers correctly the equation on the screen that Dr. Gablehauser switches on, but Sheldon retracts and Dr. Gablehauser states that what Dimitri said was indeed the correct answer and because of Sheldon's retracting, this causes Leonard, Howard, Raj and Leslie's team PMS (Perpetual Motion Squad) to win the contest.
 * Mike (Andrew Walker): an ex-boyfriend of Penny's who first appears in "The Nerdvana Annihilation" where he is shown to be dating Penny. In "The Tangerine Factor", Penny breaks up with him after he posted several "details about their sex life" in his blog. She attempted to reconcile with him after Leonard unintentionally convinced her to do so, only to walk in on him having sex with another woman. After these events, Leonard took his chance and asked Penny out on a date.
 * Blain (Tyler Olson) and Tom (Mark Hames): A couple of guys Sheldon tries to fix up with Penny in order to help break her video game addiction. In his clumsiness, Sheldon gives Blain the incorrect impression that he is inviting him on a date (with sex) rather than fixing him up on a date with a woman (Penny). Tom is a person whom Sheldon finds in an online dating service who was matched to Penny because he likes the woman to be the sexual seducer.
 * Octavia (Octavia Spencer): a clerk working at the California Department of Motor Vehicles who hands Sheldon a drivers permit after he refuses to take the test, but instead points out inaccuracies in the test. The name of the character is not used on the show, only in the credits.
 * Maria (Elena Campbell-Martinez) and Lourdes (Livia Treviño) a couple of janitors working the night shift at Cal Tech, who discover that Sheldon is living there. Sheldon instructs them not to tell anybody about it. The name of the characters is not used on the show, only in the credits.
 * Kathy O'Brian (Emily Happe): A female graduate student, who becomes Sheldon's groupie and Sheldon exploits to get free food. Kathy is the second groupie that comes into Sheldon's life after he kicked Ramona out.
 * Dr. David Underhill (Michael Trucco): A MacArthur Genius Grant recipient, David is an experimental physicist like Leonard, who is excited to be working with him. With his leather jacket and handsome looks, Penny is shocked to know that David is a scientist and starts dating him. They break up when she finds out he is married.
 * Alicia (Valerie Azlynn): A woman who moves into an apartment above Leonard and Sheldon. Penny displays a fit of jealousy at this turn of events, thinking that Alicia is supplanting her in the boys' attention. Alicia is an actress, although more successful than Penny, having landed a role on CSI as a (dead) prostitute. Eventually, to Howard's delight, she and Penny get into a catfight.
 * Mikayla (Jodi Lyn O'Keefe): a prostitute in "The Vegas Renormalization" episode whom Leonard and Raj pay to have sex with Howard.
 * "Captain Sweatpants" (Ian Scott Rudolph) and "Lonely Larry" (Owen Thayer): Two dorks that are sometimes seen at the guys' favorite comic book store. Captain Sweatpants is a middle-aged bald man who wears grey sweatpants and a City of Heroes T-shirt. Lonely Larry wears a brown suit and is extremely thin. They are also friends with Wil Wheaton.
 * Dr. Crawley (Lewis Black): An entomologist at the University, Dr. Crawley is visited by Sheldon, Howard, and Raj in order to identify the species of a cricket they found. He is agitated at losing his position at the university, his lab, and, as a result of an extended research trip, his wife.
 * Bethany (Molly Morgan), Sarah (Sarah Buehler), and Skeeter (Andy Mackenzie): Characters who appear in The Gothowitz Deviation episode. Bethany and Sarah are a couple of girls that Wolowitz and Raj meet in a Goth club. They then go to a tattoo parlor, where Skeeter is the tattoo artist. The name "Skeeter" is not used on the show, but appears in the credits.
 * Professor Laughlin (Oliver Muirhead): Professor Laughlin is a British professor who leads Cal Tech's Stellar Evolution Research Team, which Raj attempts to join in an effort to avoid being deported back to India. Unfortunately, he doesn't get the job, because an attractive female member of the team turns up at his interview, and when he attempts to cure his selective mutism by drinking some sherry he was offered, he has too much and makes an overtly sexual comment.
 * Dr. Catherine Millstone (Elizabeth Bogush): Member of Professor Laughlin's Stellar Evolution Research Team whose appearance causes Raj to drink a little too much in order to talk to her. He ends up offending both of them, thus ruining his chances of joining the team.
 * Abby (Danica McKellar) and Martha (Jen Drohan): With Leonard and Howard busy on a double date with Penny and Bernadette, Raj and Sheldon attend a university mixer where they meet Abby and Martha. Abby takes a liking to Raj, while Martha tries to connect with Sheldon. While Raj and Abby end up kissing, Sheldon completely ignores Martha, who even tries to go to bed with him.
 * Officer Hackett (Julio Oscar Mechoso), a police officer who comes to investigate the burglary in Sheldon and Leonard's apartment in The Bozeman Reaction episode. His name is not mentioned in the episode itself, but does appear in the credits.
 * Sandy (Yeardley Smith): A bureaucrat who interviewed Sheldon for a menial job in The Einstein Approximation episode. The name "Sandy" was not used on the show, but appeared in the credits.
 * Glen (Kevin Brief): A mall security guard stationed outside a ball pit in The Einstein Approximation episode. The name "Glen" was not used on the show, but appeared in the credits.
 * Venkatesh Koothrappali (Frank Maharajh): Raj's cousin, a lawyer in India, who tries to negotiate a deal for Raj's part of the One Ring prop from Lord of the Rings. He, by his own admission, is a bad negotiator, and doesn't get Raj even one of the two JetSkis that Raj wanted.
 * Dr. Elizabeth Plimpton (Judy Greer): A renowned cosmological physicist from Princeton University known for her work in quantum cosmology who comes as a personal guest of Sheldon in The Plimpton Stimulation episode, and winds up having sex with Leonard and Raj, and expresses a desire for a foursome with Leonard, Raj, and Wolowitz.
 * Sebastian (Steven Yeun) Sheldon's former roommate who left Sheldon on bad terms. Sebastian makes a single appearance in The Staircase Implementation, where he tells Leonard to "run fast, run far" from Sheldon. His name is not used in the episode, but is listed in the credits.
 * Louie/Louise (Ajgie Kirkland): A former occupant of the apartment in which Penny lives. He is a cross transvestite whom Leonard mistakenly runs into in The Staircase Implementation episode. The name Louie is used by Sheldon, referring to him/her as Louie/Louise.
 * Joyce Kim (Ally Maki): A previously unseen character who appears in The Staircase Implementation, Joyce Kim used to be mentioned as a former girlfriend of Leonard whose relationship with her only lasted a month, after which she defected back to North Korea. However, in The Staircase Implementation, Leonard admits that she wasn't a girlfriend, but a North Korean spy who attempted to get secrets from him by seducing him.
 * Mrs. Gunderson (Lauri Johnson): Third floor neighbor of Leonard and Sheldon who hears Leonard and Penny having sex and feels compelled to comment on it.
 * Joy (Charlotte Newhouse): An exercise-freak woman whom Leonard meets on a blind date set up by Bernadette via the "Girlfriend Pact" with Howard. Although Leonard finds Joy's personality to be repulsive, he agrees to go out with her again based on hints she gives that it will be easy to have sex with her.
 * Mrs. Fowler (Annie O'Donnell): Amy's strict and controlling mother. She appeared in a webcam conversation with Amy and Sheldon in which they lie to her about the nature of their relationship, telling her that they are in a romantic relationship. As a parent, she made a deal with her daughter that she date at least once a year. She has also forbidden Amy having friends and fun and even letting her pierce her ears which is why Amy was lonely and only her mother signed her high school yearbook, writing "Dear Amy, A hymen and self-respect are better than friends and fun. Love Mom.".
 * Special Agent Angela Page  (Eliza Dushku): an FBI agent investigating Howard Wolowitz's background for a security clearance in The Apology Insufficiency episode. While interviewing the guys, Raj vomits on her shoes, Leonard hits on her, and Sheldon reveals too much information about Howard, and to add insult to injury reveals too much information about Leonard.
 * Joey (Eric Andre): Theater employee who appears in "The 21-Second Excitation" when he tells Leonard, Sheldon, Howard and Raj that the theater is full up and all seats are taken. He is also a big fan of Wil Wheaton and lets him and his friends right in when they arrive instead of leaving them in the queue.
 * Professor Glenn (Rick Fox): A former professor of Bernadette's, whom she reveals to Howard that she dated for a year. Tall, dark and handsome, Glenn is physically the polar opposite of Howard, leading the insecure Howard to feel like he'll never measure up (literally AND metaphorically).
 * Mrs. Latham (Jessica Walter): A wealthy widowed benefactor who takes a shine to Leonard at a donor/faculty get-together. She asks him out with the implication that he have sex with her in order to get the lab equipment his department needs. He at first resists, but after she says that she is going to fund his project anyway, he relents. Everyone at the University congratulates him for selling himself out for the money.
 * Todd Zarnecki (Christopher Douglas Reed): Appears in "The Zarnecki Incursion" episode as a hacker who "steals" Sheldon's possessions in the World of Warcraft online game. Being physically much larger than the four main characters, they back down from confronting him, but Penny sets Todd straight with a swift kick to the groin.
 * Ho-Jun (Arnold Chun) and his dispatcher (Tom Yi): Appear in "The Agreement Dissection". Ho-Jun is a taxi driver who takes Bernadette, Penny, Amy, and Sheldon to a dance club. Communicating in Korean, Ho-Jun ask his dispatcher (who is not seen but only heard) for a place to dance waltz. Since the dispatcher does not know what is waltz, Ho-Jun has to sing it to him (after Sheldon sang it).
 * Alice (Courtney Ford): A beautiful comic book artist whom Leonard meets at the comic book store and ends up spending time with her and sharing one kiss while being in a relationship with Raj's sister Priya. When Leonard says all this, Alice kicks him out of her apartment. Later, Leonard confesses everything to Priya, but when she tells that a couple of weeks ago she slept with her ex-boyfriend, their relationship ends.
 * Eric (Eric Petersen): A guy Sheldon meets in "The Locomotive Manipulation" on the train while on a romantic Valentine's Day trip with Amy, Howard and Bernadette. They start to bond over trains and later Sheldon leaves with him and when they return from hanging out alone together, Eric detects a little friction between Sheldon and Amy after they start arguing and he doesn't want to be a third rail. After Amy makes him leave, Sheldon is made by Amy to be romantic to her and he finally kisses her on the lips on his own accord which lasts for 11 seconds and invites her to come with him to the front of the train, which she accepts and when they head for their destination, Eric follows and catches up with them.
 * Dr. Oliver Lorvis (Billy Bob Thornton): A client of Penny's who exhibits some of the same traits of each of the guys. His women dating techniques mirror Howard's creepiness, he lives with his mother and thinks that every women who is nice to him wants to be with him also like Howard. He appreciates Sheldon's way with women. He counts the length of a woman's touching him with Missisppi's like Leonard did counting duration of the hug that Penny gave him. In "The Misinterpretation Agitation", the good doctor shows up at Penny's door with flowers to initiate a romantic relationship after she winked at him and held his arm during a sales call. Sheldon notes his present to Leonard who goes to investigate. Sheldon explains that she is engaged to him and the doctor walks away upset, so Sheldon invites him in for a hot beverage. He explains that he is a urologist to the stars and has accumulated an impressive movie memorabilia collection. The guys get an invitation to see his collection which is like being in geek heaven. Sheldon remarks that the Leonard-Penny relationship is doomed, so the doctor locks them in his trophy room and goes after Penny again. walking up the stairs, he meets and connects with Amy falling for her. Penny has them both in and tries to get him to understand how creepy his actions are. Learning about the guys being locked up, she drags him back to his house with him asking about Bernadette on the way out. After the girls, rescue the guys, they can't break away from Leonard's record setting "Donkey Kong". The good doctor then shows the girls Tobey Maguire's prostate sonogram.
 * Jeanie (Kara Luiz): Howard's second cousin whom he lost his virginity to when he was 15 years old in her father's Toyota Corolla after the funeral of Howard's uncle Murray. In "The Prom Equivalency", Stuart brings her as his date to the redo prom, which humiliates Howard. Jeanie's having sex with Howard that night doesn't matter to her because it was a long time ago, but Howard soon loses it and Howard ends up attacking Stuart in the limo. During the prom, Stuart and Jeanie are about to have their picture taken, but a call to Stuart from Debbie makes Stuart leave and causes Jeanie to be angry at Stuart leaving her.
 * Josh Wolowitz (Matt Bennett): Howard's half-brother who first appears in "The Fortification Implementation" when he comes to Howard's late mother's house after learning through a lawyer that his father's name was still in the title for the house. He is a college student in San Diego studying oceanography. Howard, at first, wanted him to leave because he couldn't stand the thought of his father having another family, but then they bonded over Howard being an astronaut and their similarly warped personalities. Howard hugged Josh, telling him that he definitely is his brother.
 * Mandy Chao (Melissa Tang): A marine biologist at Caltech who was on the North Sea expedition with Leonard and at that time both were drunk and accidentally made out. Leonard confesses this to Penny 2 years later and nearly has his wedding day blown and he and Penny later end up fighting. Mandy appears in "The Separation Oscillation" where Leonard asks her if she could to talk to Penny to sort things out and during the talk, Mandy turns out to be a good therapist and a good marriage counsellor for Leonard. Later, Leonard and Penny reconcile.
 * Dr. Gallo (Jane Kaczmarek): A psychiatrist who appears in "The Sales Call Sublimation" where Penny is unable to pitch drugs to her so she sends Leonard as a patient where she is familiar with his mother's books and tells him that she doesn't agree at all with her theories on child-raising which pleases Leonard. Leonard successfully gets Dr. Gallo to meet Penny and Penny now feels that Leonard is the best husband ever. She also ends up conducting therapy on both Leonard and Penny.
 * Constance/Meemaw (June Squibb): Sheldon's maternal grandmother who is mentioned several times in the series before first appearing in the Season 9 episode "The Meemaw Materialization" where she visits Sheldon in Pasadena and also meets Leonard, Penny and Amy. It appears she came to visit her grandson, but the real reason she came was to size Amy up and she dislikes Amy because of her breaking Sheldon's heart when she broke up with him. Meemaw keeps this to herself until dinner where she reveals this in front of her grandson, his girlfriend and the married Hofstadter couple. Later, Sheldon reasons with her about how she made her husband (Sheldon's maternal grandfather) a better person when he was stubborn and egotistical and Meemaw soon gives her blessings to Sheldon and Amy's relationship.
 * Trevor (Blake Anderson): A guy with whom Sheldon has conflict when he cuts in front of him, Leonard, Howard and Raj at the movie theatre.
 * Susan (Katey Sagal): Penny's mother who first appears in the Season 10 premiere "The Conjugal Conjecture" when she comes to attend her daughter's re-wedding to Leonard.
 * Randall (Jack McBrayer): Penny's drug dealing brother who first appears in the Season 10 premiere "The Conjugal Conjecture" when he comes to attend his sister's re-wedding to Leonard. He has been in and out of prison and a bit of trouble over the years.
 * Mrs. Petrescu (Michelle Arthur): A Romanian woman who lives in the building of the Apartment building and is guest at Sheldon and Amy's brunch.
 * Isabella Concepcion (Maria Canals-Barrera): Caltech janitor who Raj has a crush on. She comes into the telescope room cleaning and then they start cleaning the men's room together. Another night Isabella comes into the telescope room to work and has found that Raj has made them dinner since she was too busy to go out. He even cleaned up the next offices so that she could take a break. He made what he claimed was food from her homeland; however she is Cuban and not Mexican. They talk about when they first came to America. She mentions that she has some time off on Sunday as Howard comes in. He asks if she was the astronomer that Raj met. Isabella is not happy and leaves because she is not going to waste her time with someone who is embarrassed by her. Raj runs after her to apologize and he tells her embarrassing things about him. Finally she says that he can take her out to a nice place to eat on Sunday night and it can be Pakistani food if he likes. He says he's Indian, to which Isabella says now he knows what it's like to be stereotyped.
 * Theodore (Christopher Lloyd): An old man who temporarily rents Sheldon's old room in the tenth episode of Season 10 "The Property Division Collision" whom Sheldon uses to get back at Leonard and after Leonard really loses and gets enormously, extremely and incredibly angry with Sheldon, Theodore gave both of them good advice on why they were fighting, because it is hurting them to leave each other. He is never seen again after this and no longer lives in Apartment 4A as of episodes 14 and 18.

Multiple scenes / multiple episodes

 * Summer Glau (Season 2, "The Terminator Decoupling"): When the guys travel by train to a conference in San Francisco, they realize Summer Glau (who was in two science fiction television shows, Firefly and Terminator: The Sarah Connor Chronicles) is sitting in the same passenger car. Raj, Howard, and Leonard take turns talking with her. Raj can't talk without drinking beer (which turned out to be non-alcoholic), Howard is his usual creepy, over-the-top self, and Leonard can't start a conversation before she has to get off the train.
 * Wil Wheaton first appeared in the season 3 episode "The Creepy Candy Coating Corollary". See his recurring character section for more information.
 * Katee Sackhoff (Season 3, "The Vengeance Formulation" and Season 4, "The Hot Troll Deviation"): Howard fantasizes about taking a bath with Katee Sackhoff (Captain Kara "Starbuck" Thrace in Battlestar Galactica). However, in his own fantasy Sackhoff scolds him and tells him to get a real girlfriend instead of imaginary ones. Sackhoff reappeared in a different fantasy of Howard's in The Hot Troll Deviation, where she and George Takei help Howard realize that he still has feelings for Bernadette.
 * George Takei (Season 4, "The Hot Troll Deviation"): During one of Howard's fantasies, Takei, along with Katee Sackhoff, help Howard realize that he still has feelings for Bernadette. Takei's real life homosexuality is twice referenced in the episodes: once, when he first appears in Howard's fantasy, Sackhoff asks Howard if Takei's presence implies that Howard has homosexual tendencies, and the second reference occurs when Takei advises Howard about women's love preferences. Sackhoff asks Takei "How would you know?" and he responds "I read."
 * Neil deGrasse Tyson (Season 4, "The Apology Insufficiency") Appears as a colleague of Raj. Upon being introduced to deGrasse Tyson, Sheldon tells him that he (Sheldon) is upset at Tyson's role in the demotion of Pluto from planet status. Initially deGrasse Tyson explains that he had no role in the demotion, but later attempts to apologize to Sheldon. Sheldon declines the apology, immediately after he himself apologized to Howard, who declined Sheldon's apology.
 * Stephen Hawking (Season 5, "The Hawking Excitation"): When Sheldon finds out that Howard gets a chance to work with Stephen Hawking, he tries to get Howard to let him meet Stephen Hawking so that he can present his idea on string theory to him. Howard, however, asks Sheldon to do all sorts of tasks for him, (cleaning his belt buckles, wearing a French maid costume at the cafeteria, shopping with his mother) before agreeing to help him meet Hawking, which Sheldon reluctantly agrees. When Sheldon finally meets Hawking, Hawking points out that Sheldon has done an arithmetic error on the paper he gave him, which causes Sheldon to gasp and faint. He also criticized Leonard and Sheldon's theory anonymously, played "Words with Friends" with Sheldon and got prank phone calls form him.
 * James Earl Jones (Season 7, "The Convention Conundrum"): With the boys acting like "teenagers" about Comic-Con tickets and Sheldon is having the best day ever with James Earl Jones, the girls decide they're going to have a more grown up afternoon and go out for tea, but then they end up realizing they don't feel very grown up either.[1]
 * Adam Nimoy (Season 9, "The Spock Resonance"): He and Wil Wheaton interview Sheldon for an interview on Mr. Spock.
 * Elon Musk (Season 9, "The Platonic Permutation"): Howard meets him at a kitchen when he, Bernadette, Raj and Emily serve homeless people and it turns out Howard is a big fan of him.
 * Adam West (Season 9, "The Celebration Experimentation"): When Leonard, Penny and Amy find out that Sheldon dislikes birthday parties because when he was a child, his sister and her friends used to torture him and on the sixth birthday, they tricked him into believing that Batman was coming to the party, to which Sheldon waited by he door for hours, but the closest thing he saw to Barman was when a robin flew into the window. Stuart once had him in his comic-book store and he gives Leonard Adam West's contact information and he is brought to Sheldon's birthday party.

Cameo appearances

 * Charlie Sheen (Season 2, "The Griffin Equivalency"): When Raj discovers a celestial body he is very excited about it. At Penny's restaurant, Raj says to the patron at the next table over, "Hey, buddy. I'm going to be in People magazine!"; Charlie Sheen turns around and says "Yeah? Call me when you're on the cover."
 * Analeigh Tipton and Samantha Potter (Season 2, "The Panty Piñata Polarization"): Howard and Raj visit the America's Next Top Model house pretending to be cable television repairmen. Tipton and Potter made appearances, opening the door for Howard and Raj. Tipton greeted Howard and Raj, caught them in a lie as the house does not receive cable television (it receives satellite), but allows them in regardless when Howard "corrects" his error with another lie when he says that they are satellite television repairmen. Potter did not get a speaking role. She shows up again playing Vanessa Bennet, a woman who shares Sheldon's interests and solved a complicated puzzle to meet him. Unfortunately she was too late and Sheldon rejected her.
 * George Smoot (Season 2, "The Terminator Decoupling"): After a train ride to a conference in San Francisco, Sheldon presents his paper to 2006 Nobel Prize laureate George Smoot, and proposes joint research; Smoot abruptly rejects his idea by asking "With all due respect, Dr. Cooper, are you on crack?"
 * Ira Flatow (Season 3, "The Vengeance Formulation"): Sheldon is invited to talk about magnetic monopoles on Flatow's radio show, Science Friday. However, Barry Kripke pulls a prank on Sheldon, and Sheldon is publicly humiliated to a nationwide audience. Flatow's appearance was voice only. Later he appeared in person to interview Leonard and Sheldon.
 * Stan Lee (Season 3, "The Excelsior Acquisition"): When Sheldon has to miss Lee's appearance at the comic book store because he ends up in traffic court after running a red light, Penny tries to make it up to him by taking him to Lee's house, uninvited and unannounced (acquiring Lee's address from Stuart). Lee is furious and sarcastically tells Penny and Sheldon to watch a basketball game with him, but Sheldon thinks Lee is serious and rushes into Lee's house. When Sheldon announces his "autographed" restraining order to Leonard, Howard, and Raj, he comments it will look great hanging next to his restraining order from Leonard Nimoy.
 * Steve Wozniak (Season 4, "The Cruciferous Vegetable Amplification") Dines in Penny's restaurant at the same time that the guys are there. He compliments Sheldon for his "virtual presence device" (using Sheldon's name for the device without having heard how Sheldon calls it) and in turn Sheldon (having previously referred to him as "the Great and Powerful Woz") compliments Wozniak telling him that he ranks 15th on his list of favorite technological visionaries. When Wozniak feigns disappointment, Sheldon tries to console him by stating that the ranking is six places ahead of Apple founder Steve Jobs. Sheldon then tells Wozniak that the Apple II was rather 'nifty' and is one of his proudest possessions. Wozniak promises Sheldon that he would autograph the computer if Sheldon brought it to him. In the haste to get the Apple II to Wozniak for signing, Sheldon trips and falls down the stairs, breaking the computer.
 * LeVar Burton first appeared in the season 4 episode "The Toast Derivation". See his recurring character section for more information.
 * Brian Greene (Season 4, "The Herb Garden Germination"): Sheldon and Amy attend a book signing event of Greene's popular science book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos. At the event, Sheldon initially ridicules Greene in private conversations with Amy, but then he gets up and publicly ridicules Greene, asking Greene why doesn't he do something more useful with his time than write popular science (only to turn around and state he was kidding).
 * Brent Spiner (Season 5, "The Russian Rocket Reaction"): Sheldon became torn about attending a party organized by his mortal enemy Wil Wheaton, when he finds out that Brent Spiner will be attending. Despite his stubbornness, Leonard and Raj had no trouble going and Sheldon eventually attends the party, by reasoning out that he wants to salvage his friendship with Leonard, but really, he wants to meet Brent Spiner. He eventually made up with Wil Wheaton, who presents him with his long-desired Wesley Crusher's mint-in-condition action figure. Brent Spiner accidentally rips open the packaging, landing him in Sheldon's mortal enemies list. He also made a deal with Leonard to attend Leonard's birthday party.
 * Leonard Nimoy as "Spock" (Season 5, "The Transporter Malfunction"): When Penny presents Leonard and Sheldon with mint-in-condition Star Trek Transporter, Sheldon gets persuaded by his Tiny Spock toy (voiced by Nimoy) to open the present and play with it. Sheldon does it and the transporter gets broken as a result. He secretly switches his broken transporter with Leonard's. only to be haunted by guilt in the form of Tiny Spock appearing in his dream and persuading him to tell the truth. Sheldon eventually does so, to get rid of his guilt.
 * Howie Mandel (Season 6, "The Re-Entry Minimization"): Actor who makes a short appearance after Howard returns from space.
 * Carrie Fisher (Season 7, "The Convention Conundrum"): James Earl Jones and Sheldon go to her house and run off, after ringing her doorbell. Fisher came out, holding a baseball bat and yelled, "It's not funny anymore, James!" and then goes back inside, after hearing him laughing.
 * Nathan Fillion (Season 8, "The Comic Book Store Regeneration): Leonard and Raj have their picture taken with him.
 * Kevin Smith (Season 8, "The Fortification Implementation): Actor whose voice makes a brief appearance before Penny's live podcast show in her apartment.
 * Ellen DeGeneres (Season 10, "The Geology Elevation): Sheldon and Bert go to her show together.