John Mark "Johnny" Galecki (born April 30, 1975) is an American actor. He is known for his role as David Healy in Roseanne from 1992–97 and as Dr. Leonard Hofstadter in The Big Bang Theory since 2007. Galecki also appeared in the films National Lampoon's Christmas Vacation (1989), Suicide Kings (1997), I Know What You Did Last Summer (1997), Bookies (2003), Hancock (2008) and In Time (2011).