Jim Parsons as Sheldon Cooper
Parsons first credited TV role was in 2002 on an episode of Ed, while his first movie role was as Tim on Garden State. He most recently starred on HBO's The Normal Heart and earned an Emmy nomination. He's won four Emmys for his portrayal of Sheldon Cooper.
Jim Parsons on Judging Amy
Jim Parsons portrayed Rob Holbrook on 7 episodes of CBS's Judging Amy.
Johnny Galecki as Leonard Hofstadter
Galecki's first role was on a TV movie, Time Out for Dad in 1987. He followed that up with small roles on several movies, before getting back into television. He played Danny Nash on American Dreamer and David MacGregor on Billy before getting his first well-known role on the comedy, Roseanne.
Johnny Galecki on Roseanne
Johnny Galecki portrayed David Healy, Darlene's boyfriend, on Roseanne.
Kaley Cuoco-Sweeting as Penny - The Big Bang Theory
Cuoco-Sweeting's first role was on a TV movie, Quicksand: No Escape in 1992. She also played a young Angela on My So Called Life. Her big break came as Bridget Hennessy on
8 Simple Rules, which she followed up as Billie Jenkins on Charmed.
Kaley Cuoco-Sweeting on 8 Simple Rules
Kaley Cuoco-Sweeting portrayed Bridget Hennessy on 8 Simple Rules.
Simon Helberg as Howard Wolowitz
Helberg's first role was in 1999 as the College Roommate on the movie, Mumford. He went on to play Seth on a few episodes of TV's Joey and then as Alex Dwyer on Studio 60 on the Sunset Strip.
Simon Helberg on Popular
Simon Helberg played Gus Latrine on an episode of Popular for his first credited television role.
Kunal Nayyar as Raj Koothrappali
Nayyar's first role was in the 2004 moive, S.C.I.E.N.C.E, where he played Pizza Man. On television, he was first seen as terrorist on NCIS.
Kunal Nayyar on NCIS
Kunal Nayyar had a small role as Youssef Zidan on NCIS. Gibbs took him down!
Mayim Bialik as Amy Farrah Fowler
Bialik's first role was on the original Beauty and the Beast playing a young girl, Ellie. She went on to be cast on some of the best shows of the day, including Facts of Life, Webster, and MacGyver. In 1990, she took on the titular role of Blossom Russo on Blossom. She joined The Big Bang Theory in Season 3 as Amy.
Mayim Bialik on Beauty and the Beast
Mayim Bialik's first credited role was on the original Beauty and the Beast on CBS.
Melissa Rauch as Bernadette Rostenkowski
Rauch's first role was in 2006 as Megan in the movie, Delirious. In 2010, she had a short run on
True Blood as Summer. She joined The Big Bang Theory in Season 3 as Bernadette.
Melissa Rauch on True Blood
Melissa Rauch played Summer on True Blood. She was in love with Hoyt and had his mother's support.