Included among them are parents of the main characters, their dates and their coworkers.
The American sitcom The Big Bang Theory, created and executive produced by Chuck Lorre and Bill Prady, premiered on CBS on September 24, 2007.
The series initially centers on five characters: roommates Leonard Hofstadter and Sheldon Cooper, two Caltech physicists; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's friends and co-workers aerospace engineer Howard Wolowitz, and astrophysicist Raj Koothrappali.
Over time, several supporting characters have been introduced and promoted to starring roles, including Bernadette Rostenkowski-Wolowitz, Amy Farrah Fowler, Stuart Bloom, Emily Sweeney, and Leslie Winkle.
The series also features numerous supporting characters, each of whom plays a prominent role in a small group of episodes.
Following his breakup with Penny, Leonard began a relationship with Raj's sister Priya in season 4, and it is implied they had a brief sexual encounter before the start of the series on an occasion when Priya visited Raj.
The two attempted a long-distance relationship after Priya moved back to India, but Leonard eventually broke up with her in season 5 after she admitted to cheating on him with her former boyfriend.
In season 5's "The Beta Test Initiation", Leonard and Penny start dating anew though their relationship is rocky.
Compared to his friends he is relatively successful with women.
The writers have toyed with a romance between him and next-door neighbor Penny since the pilot, with their unresolved sexual tension being a major force for drama.