Johnny Galecki

Johnny Galecki plays “Leonard Hofstadter” on the CBS series The Big Bang Theory. He is most known for his role of “David Healy” on the ABC sitcom Roseanne as well as his portrayal of "Rusty Griswold" alongside Chevy Chase, Beverly D’Angelo, and Juliette Lewis in National Lampoon’s Christmas Vacation. Galecki has appeared in Will Smith’s Hancock and on the television series My Boys, American Dad!, Hope & Faith, and “My Name is Earl.”
He starred with Justin Timberlake and Amanda Seyfried in the film, In Time and played "Johnny Galecki" on several episodes of Entourage.

More about the show at the official The Big Bang Theory site.