Johnny Galecki
John Mark "Johnny" Galecki (born April 30, 1975) is an American actor best known for his role as Dr. Leonard Hofstadter in The Big Bang Theory since 2007 and as David Healy in Roseanne from 1992–97. He 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).