<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 since 2007 and David Healy in the ABC sitcom Roseanne from 1992 to 1997 . In 2018 , he appeared in the revival of the sitcom reprising his role as David . 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 since 2007 and David Healy in the ABC sitcom Roseanne from 1992 to 1997 . In 2018 , he appeared in the revival of the sitcom reprising his role as David . 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>
who played leonard in the big bang theory
who played leonard in the big bang theory
John Mark Galecki
John Mark Galecki