Simplify the following text for a Grade 7 native English-speaking person.
Original: Galecki is best known for playing the role of David Healy on Roseanne and Leonard Hofstadter on The Big Bang Theory .
Simplified: He is best known for his roles as David Healy in the sitcom Roseanne , Rusty Griswold in National Lampoon 's Christmas Vacation , and as Leonard Hofstadter , Ph. . D , in the CBS sitcom The Big Bang Theory .