John Mark Galecki (born April 30, 1975) is an American actor. He played Leonard Hofstadter in the CBS sitcom The Big Bang Theory (2007–2019) 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).
Galecki was one of the highest paid television actors in the world, with his role in The Big Bang Theory earning him approximately US$900,000 per episode between 2017 and 2019. In 2018, he was estimated to be the world's second highest-paid TV actor by Forbes, earning $25 million.