<P> John Mark Galecki ( born April 30 , 1975 ) is an American actor . He is known for playing Leonard Hofstadter in the CBS sitcom The Big Bang Theory ( 2007 -- present ) and David Healy in the ABC sitcom Roseanne ( 1992 -- 1997 ; 2018 ) . 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 ) , In Time ( 2011 ) , and Rings ( 2017 ) . </P>
<P> John Mark Galecki ( born April 30 , 1975 ) is an American actor . He is known for playing Leonard Hofstadter in the CBS sitcom The Big Bang Theory ( 2007 -- present ) and David Healy in the ABC sitcom Roseanne ( 1992 -- 1997 ; 2018 ) . 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 ) , In Time ( 2011 ) , and Rings ( 2017 ) . </P>
the guy who plays leonard on big bang theory
the guy who plays leonard on big bang theory
John Mark Galecki
John Mark Galecki