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 shares 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. 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 two other accomplished scientists: his mother, Dr. Beverly Hofstadter, and sister, while his brother is a Harvard law professor. Leonard wears glasses and is lactose intolerant.
 * 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 calculating, cynical, and 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 mother, Mary, is a devoted Christian, and his twin sister, Missy, is certainly no scientist. Sheldon is consistently the geekiest 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.
 * Penny (family name is not revealed) (Kaley Cuoco), (appeared in all but two episodes in season 4, but credited for the episodes in which she did not appear) is Leonard and Sheldon'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 have since broken up. In the most recent episodes, she has shown to be influenced by her geek friends, including an obsession with on-line 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 good friends.
 * 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 Raj, 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.
 * 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. 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.
 * Dr. Bernadette Maryann Rostenkowski-Wolowitz (Melissa Rauch) 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 Engagement Reaction. Bernadette has a squeaky voice and when she is angry, she sounds exactly like Mrs. Wolowitz, and Mrs. Wolowitz 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. The two eventually got married on The Countdown Reflection, shortly before Howard left for space.
 * Dr. Amy Farrah Fowler (Mayim Bialik) 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, and she has an Aunt Flora who is 93 years old. Amy's family has been described to be very nosy about her romantic life.
 * Stuart Bloom (Kevin Sussman): Stuart 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 mistakenly 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.

Recurring characters
These characters appear in several episodes. The list is sorted by chronological order of appearance, considering their first appearance in the show.
 * 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): A devout Christian from Texas, Mary is Sheldon's mother. She has two other children besides Sheldon; Sheldon's fraternal twin sister, Missy and his brother George, who is shown via a family tree in "The Cruciferous Vegetable Amplification" to be three years older than Sheldon and Missy. She appeared in the fourth episode of the first season, the season three premiere, and the third episode of season four. 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. 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.
 * Mrs. Wolowitz (voiced by Carol Ann Susi): Howard's overbearing 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. Mrs. Wolowitz 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.
 * 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 the "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 gynaecologist and drives a Bentley. The Koothrappalis enjoy Doogie Howser reruns, which are apparently new to India.
 * 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.
 * Dr. Beverly Hofstadter (Christine Baranski): Leonard's overly analytical mother, Beverly is a neuroscientist as well as a psychiatrist. 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 most recent appearance that the two are 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 both 2009 and 2010, Christine Baranski was nominated for an Emmy Award for Outstanding Guest Actress in a Comedy Series for this role.
 * 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.
 * Wil Wheaton (portraying an 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.
 * Zack Johnson (Brian Thomas Smith): A dim-witted 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.
 * Priya Koothrappali (Aarti Mann): Raj's 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 into signing it.

Minor characters
These characters appear in fewer episodes, most in just one. 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 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). The name of the character is never used on the show, only on her nametag and in the credits.
 * 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.
 * Chen (James Hong): The owner of a Chinese restaurant the guys frequently visit.
 * Christy (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). Mrs. Wolowitz 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.
 * 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.
 * 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.
 * Ramona Nowitzki (Riki Lindhome) and Kathy O'Brian (Emily Happe): A couple of female graduate students, who become Sheldon's groupies and Sheldon exploits to get free food. Ramona is the first groupie who meets Sheldon after he introduced his work to 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. Kathy 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 young blonde 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 Randolph) and "Lonely Larry" (Owen Thayer): Two nerds 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 dresser whom Leonard mistakenly runs into in The Staircase Implementation episode. The name Louie is used by Sheldon, referring to him/her as Louie/Louise.
 * 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 Farrah Fowler's 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.
 * Special Agent Angela Paige (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.
 * Wyatt (Keith Carradine): Penny's father. 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.
 * 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).
 * 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.
 * 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 take Bernadette, Penny, Amy, and Sheldon to a dance club. Communicating in Korean, Ho-Jun ask his dispatcher (who is not seen) 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).

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.

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.
 * 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.
 * 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 (Season 4, "The Toast Derivation"): Sheldon invited Burton to a party via a tweet. 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.
 * 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 ogranized 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.
 * 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 cafetaria, 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.