John Mark Galecki (born April 30, 1975) is an American actor. He is known for playing David Healy in the ABC sitcom Roseanne from 1992 -- 1997 and Dr. Leonard Hofstadter in the CBS sitcom The Big Bang Theory since 2007. Galecki also appeared in the films National Lampoon 's Christmas Vacation (1989), Prancer (1989), Suicide Kings (1997), I Know What You Did Last Summer (1997), Bookies (2003), and In Time (2011).
who is johnny galecki on big bang theory
Dr. Leonard Hofstadter