The Big Bang Theory is an American sitcom created by Chuck Lorre and Bill Prady, both of whom serve as executive producers on the show. It premiered on CBS on September 24, 2007.

Set in Pasadena, California, the show is about two Caltech prodigies in their 20s, one an experimental physicist (Leonard Hofstadter) and the other a theoretical physicist (Sheldon Cooper), who live across the hall from an attractive blonde waitress with show-biz aspirations (Penny). Leonard and Sheldon's 'geekiness' and sheer intellect are contrasted for comic effect with Penny's social skills and common sense. Two equally geeky friends of theirs, Howard and Rajesh, are also main characters.

The show is produced by Warner Bros. Television and Chuck Lorre Productions. In March 2009, it was reported that The Big Bang Theoryhad been renewed for a third and fourth season by CBS. In August 2009, the sitcom won the best comedy series TCA award and Jim Parsons won the award for individual achievement in comedy.

When the third season premiered on September 21, 2009, it ranked as CBS's highest-rated show of that evening in the adults 18–49 demographic (4.6/10), along with a then series-high 12.83 million viewers. The Big Bang Theory airs on Mondays at 9:30 EST following another Chuck Lorre-produced show Two and a Half Men

• Johnny Galecki as Leonard Hofstadter, Ph.D. – Leonard 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. He is the straight man of the series. The writers have toyed with a romance between him and neighbor Penny, with their unresolved sexual tension being a major force for drama. The third season centers around their struggle to maintain a relationship.
• Jim Parsons as Sheldon Cooper, Ph.D. – Originally from East Texas, he was a child prodigy, starting college at the age of 12, right after completing the 5th grade. As a theoretical physicist, he possesses a master's degree, two Ph.D.s, and an IQ of 187. He 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 shares an apartment with Leonard Hofstadter.
• Kaley Cuoco as Penny – She is the attractive, blonde neighbor who lives across the hall from Sheldon and Leonard. She has aspirations of a career in show business, and has been to casting calls and auditions but has not been successful thus far. To pay the bills, she works as a waitress at The Cheesecake Factory.
• Simon Helberg as Howard Wolowitz, M.Eng.– He works as an aerospace engineer and lives with his mother; they are Jewish. 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 MIT and the apparatus he designs are actually built and launched into space, unlike the purely abstract work of his friends. He provides outrageous pick-up lines and fancies himself a ladies man with suitably unimpressed reactions from Penny; however, he has shown limited success with other women.
• Kunal Nayyar as Rajesh Koothrappali, Ph.D. – Rajesh, who originally comes from New Delhi, India, works as a particle astrophysicist at Caltech. He is very shy around females and is physically unable to talk to them unless he drinks alcohol. However, he has had much better luck with women as opposed to his overly confident best friend Howard. His parents, introduced as Dr. and Mrs. V.M. Koothrappali, are seen via webcam. In the third season, he works for Sheldon because his research has run into a dead-end and he doesn't want to go back to India.
• Sara Gilbert as Leslie Winkle, Ph.D. (recurring season 1-present, starring season 2) – She is an experimental physicist who has casual sex with both Howard and Leonard during the show's airing. She does not get along well with Sheldon Cooper and frequently mocks him. She was promoted to main cast during the second season, but demoted again once the writers realized they could not produce quality material for her for every episode.