The cast of The Big Bang Theory may have transformed over the years that they spent on the show, but that's nothing compared to the difference between the stars now as adults and what they were like as children.

Born in Belgium, Johnny Galecki's father was stationed in the country at the time with the U.S. Air Force. However, it wasn't long before the family moved to the United States, so Galecki mostly grew up in Oak Park, Illinois. Even back then, he had dreams of Hollywood.

In a 2018 interview with Variety, Galecki said he'd been talking about being an actor since the age of three, despite having no family members who knew anything about acting. His family was poor, with little money to go to the movies, but the young Galecki was convinced he knew what he wanted to do with his life, and he went after his dreams.

Galecki followed his passion and started racking up credits in movies like 1989's National Lampoon's Christmas Vacation and on shows like Blossom in 1991. Of course, he also appeared on Roseanne, playing troubled teen David Healy, the boyfriend of Darlene, aka Sara Gilbert, another alum of The Big Bang Theory and someone else who was an adorable kid. Keep watching to see how The Big Bang Theory Cast Was Unrecognizable As Kids!

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Johnny Galecki | 0:11

Jim Parsons | 1:11

Kaley Cuoco | 1:48

Simon Helberg | 2:49

Kunal Nayyar | 3:34

Mayim Bialik | 4:12

Melissa Rauch | 5:20

Sara Gilbert | 6:12

Christine Baranski | 7:15

John Ross Bowie | 8:06

Wil Wheaton | 9:02