John Mark "Johnny" Galecki (born April 30, 1975) is an American (United States) actor. He is best known for his roles as David Healy (List of Roseanne characters#David Healy) in the sitcom Roseanne (Roseanne (TV series)), Rusty Griswold in National Lampoons Christmas Vacation, and as Leonard Hofstadter, Ph.D, in the CBS sitcom The Big Bang Theory.