- Johnny Galecki
- Jim Parsons
- Kaley Cuoco
- Simon Helberg
- Kunal Nayyar
- Mayim Bialik
- Melissa Rauch
- Kevin Sussman
Since its premiere in 2007, The Big Bang Theory has starred Johnny Galecki as Leonard Hofstadter, Jim Parsons as Sheldon Cooper, Kaley Cuoco-Sweeting as Penny, Simon Helberg as Howard Wolowitz and Kunal Nayyar as Raj Koothrappali.
Sara Gilbert, who had a recurring role as Leslie Winkle in Season 1, joined the cast for the second season. Sara Gilbert made her first appearance since the show's third season in the 200th episode, "The Celebration Experimentation", during its ninth season.
Melissa Rauch and Mayim Bialik, who first appeared in Season 3 as Bernadette Rostenkowski and Amy Farrah Fowler, respectively, were promoted to series regulars during The Big Bang Theory's fourth season and remain members of the cast to this day.
Kevin Sussman, who appeared frequently as Stuart Bloom during Seasons 2 - 5, was promoted to a semi-regular cast member during the show's sixth, eighth and ninth seasons. Laura Spencer, who plays Raj's girlfriend Emily Sweeney, was promoted to a semi-regular during the ninth season.
Recurring Guest Stars
Carol Ann Susi was the voice of Mrs. Wolowitz in numerous episodes from 2007 until 2014 when she passed away following a short illness. The character of Mrs. Wolowitz was subsequently written out of the show.
John Ross Bowie has appeared as Barry Kripke, an annoying colleague of the guys, in eighteen episodes since its second season.
Aarti Mann had a recurring role as Priya, Raj's sister who had a relationship with Leonard, during the show's fourth and fifth season.
Wil Wheaton has appeared in thirteen episodes as a fictionalized version of himself, initially acting as an enemy to Sheldon, since the third season.
Laurie Metcalf, who previously starred on the TV show Roseanne along side Johnny Galecki, has appeared in nine episodes as Sheldon's mother, Mary Cooper.
Christine Baranski, who previously worked with The Big Bang Theory's Chuck Lorre on his show Cybill, has appeared in nine episodes as Leonard's mother, Dr. Beverly Hofstadter.
Brian George and Alice Amter have appeared as Raj's parents in a number of episodes since the first season. In Season 8, Raj's dad visited Pasadena and was seen "in person" on the show for the first time.
Brian Thomas Smith has appeared in seven episodes as Penny's former boyfriend and one-time husband, Zack, from the third season.
Casey Sander has played Bernadette's father, Mike Rostenkowski, in six episodes of The Big Bang Theory, starting in Season 5.
Keith Carradine has appeared in two episode as Penny's father, Wyatt, starting in Season 4.