- 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 as Penny, Simon Helberg as Howard Wolowitz and Kunal Nayyar as Raj Koothrappali.
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.
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".
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 nineteen episodes since its second season.
Wil Wheaton has appeared in sixteen episodes as a fictionalized version of himself since the third season. His character was initially antagonistic to Sheldon, who considered him one of his mortal enemies, but the pair have since become friends.
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.
Laurie Metcalf, who previously starred on the TV show Roseanne alongside Johnny Galecki, has appeared in eleven episodes as Sheldon's mother, Mary Cooper. Her real-life daughter, Zoe Perry, is set to play the part of Mary Cooper in the upcoming The Big Bang Theory spin-off series, Young Sheldon.
Christine Baranski, who previously worked with The Big Bang Theory co-creator Chuck Lorre on his show Cybill, has appeared in eleven episodes as Leonard's detached 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 eight episodes as Penny's former boyfriend and one-time husband, Zack, from the third season.
Dean Norris has guest starred in five episodes of The Big Bang Theory's tenth season as Colonel Williams, who is overseeing the guys' military guidance system project.
Casey Sander has played Bernadette's father, Mike Rostenkowski, in six episodes of The Big Bang Theory, starting in Season 5.
Judd Hirsch has guest starred in two episodes of The Big Bang Theory as Leonard's father, Alfred.
Keith Carradine has appeared in three episode as Penny's father, Wyatt, first in Season 4. Katey Sagal, who played the on-screen mother of Kaley Cuoco's character in the 2002-2005 sitcom 8 Simple Rules, has appeared as Penny's mother in one episode of The Big Bang Theory. Jack McBrayer has appeared in one episode of The Big Bang Theory as Penny's brother, Randall.