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 toyed with a romance between him and neighbor Penny,with their unresolved sexual tension being a major force for drama.Leonard and Penny eventually had a short intimate relationship.
Jim Parsons as Sheldon Cooper,Ph.D.– Originally from East Texas,he was a child prodigy,starting college at the age of 11,right after completing the fifth 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.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 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 women 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.