John Mark "Johnny" Galecki (born April 30, 1975) is an American actor. He is best known for his roles as David Healy in the sitcom Roseanne, Rusty Griswold in National Lampoon's Christmas Vacation, and as Leonard Hofstadter, Ph.D, in the CBS sitcom The Big Bang Theory.

Galecki was born in Bree, Belgium, to American parents. His mother, Mary Lou, is a mortgage consultant, and his father, Richard Galecki, was a member of the U.S. Air Force stationed in Belgium and also worked as a rehabilitation teacher. Galecki moved to Oak Park, Illinois, with his family when he was three years old.

In 1989, he played Russell "Rusty" Griswold in National Lampoon's Christmas Vacation. His best known early role, though, was as David Healy (first introduced as Kevin Healy) in the U.S. sitcom Roseanne. His character was the artistically talented, curly-haired, soft-spoken boyfriend (later husband) of Darlene, the droll, tomboyish second daughter of the Conner family played by Sara Gilbert. Galecki was cast in similar roles in the 1997 summer slasher I Know What You Did Last Summer as a jealous loser who has an eye for Jennifer Love Hewitt and the 1998 dark comedy The Opposite of Sex as a gay teen who lies about being molested. In the year 2000, Galecki took a bit of a twist and reunited with the same filmmaker who cast him in Bounce as Seth (a gay man), an assistant to Ben Affleck's character Buddy Amaral, who also helps dodge out with his love life.