Dr Sheldon Lee Cooper – Sheldon Cooper, M.A., Ph.D. – Originally from Galveston, Texas, he was a child prodigy with an eidetic memory who began college at the age of 11 (after completing the fifth grade), started graduate studies at 14, and earned a Ph.D. at 16. A theoretical physicist researching quantum mechanics and string theory, he has two master’s degrees, a Ph.D., a Sc.D., and an IQ of 187. He exhibits a strict adherence to routine and a lack of understanding of irony and sarcasm; he is also uninterested in many of the romantic hijinks of his friends. Sheldon shares an apartment with Leonard Hofstadter, across the hall from Penny, and relies on both for advice in social situations. Sheldon is very egotistical, as he often boasts about his ‘superior’ intelligence, and sometimes underestimates that of his friends.
Dr Leonard leaky Hofstadter – Leonard Hofstadter, Ph.D. – An experimental physicist with an IQ of 173. He received his Ph.D. when he was 24 years old. He is originally from New Jersey. The straight man of the series, he shares an apartment with colleague and friend Sheldon Cooper in Pasadena.
Howard Joel Wolowitz – Howard Joel Wolowitz, Master of Engineering (M.Eng.) – He works as an aerospace engineer. He is Jewish and lives with his mother. Unlike Sheldon, Leonard, and Raj, Howard lacks a Ph.D. He defends this by pointing out that he has a master’s degree in engineering from the elite MIT and that the apparatus he designs are launched into space, including himself in the Season 5 finale. He exhibits a good line in Jewish humor, which Leonard and Raj seem to occasionally appreciate.
Penny – From a town outside of Omaha, Nebraska, she is an aspiring actress who lives across the hall from Sheldon and Leonard. Penny has been on casting calls and auditions but has not been very successful thus far. To pay the bills, she is a waitress and occasional bartender at The Cheesecake Factory until the seventh season. She is not a university graduate but has far more common sense and streetwise social awareness than the other main characters. There is, however, a running joke about her being slovenly and untidy. To date, her family name has not been revealed. She dated Leonard at the end of the first season, and during the third, fifth, and sixth seasons. By season four, Bernadette, Amy and Penny have formed their own group, who like to hang out in Penny’s apartment or go out together.
Dr Rajesh Ramayan ‘Raj’ Koothrappali, Ph.D. – Originally from New Delhi, India, he works as a particle astrophysicist at Caltech. His family is very wealthy. He communicates with his parents, Dr. and Mrs. V.M. Koothrappali, via webcam. He was very shy around women and was physically unable to talk to them for the first six seasons of the show (except for his mother and his sister), unless he drank alcoholic beverages (or at least thought he had been drinking alcohol), or had taken experimental medications provided by the pharmacology department at the university. However, he often had better luck with women than his overly-confident best friend, Howard. In the final episode of season six, Raj overcomes his inability to speak to women, and can now address them without having alcohol in his system. He has very feminine tastes and often takes on a stereotypical female role in his close friendship with Howard, but he insists that he is not gay.
Dr Bernadette Rostenkowski-Wolowitz, Ph.D. (recurring season 3, starring since season 4) – a young woman who is initially a waitress and co-worker of Penny’s, paying her way through graduate school microbiology studies. She defends her doctoral thesis and lands a high-paying research position at the end of season 4. Bernadette is introduced to Howard by Penny. At first they do not get along, apparently having nothing in common. When they find out that they both have overbearing mothers, they feel a connection. During season 3 they date and then break up off-screen, then get back together in season 4. They become engaged near the end of season 4, and marry at the end of season 5.
Dr Amy Farrah Fowler – Amy Fowler, Ph.D. (guest starring season 3, starring since mid-season 4) – a woman Raj and Howard met on an online dating site after secretly setting up an account using Sheldon’s name and information. The site matches her to Sheldon, and the two share many similar traits though Amy eventually becomes more interested in social and romantic interaction and is slightly more socially-aware than Sheldon. Once she and Sheldon meet, she becomes, as Sheldon puts it, a girl who is his friend, but not his “girlfriend”. Their relationship slowly progresses through seasons 5 and 6. Amy also believes she and Penny are best friends (“besties”, by her own definition), a sentiment that at first Penny respectfully indulges but does not share. Penny eventually becomes a real friend, overlooking Amy’s Sheldon-like qualities. Amy’s admiration for Penny has at times bordered on physical attraction. Amy Fowler has a Ph.D. in neurobiology, while Bialik herself has a doctorate in neuroscience; in the season 1 episode “The Bat Jar Conjecture”, Raj suggests recruiting “The girl who plays TV’s Blossom” (Bialik) to their Physics Bowl team.