See How the 'Big Bang Theory' Cast Has Changed Over 12 Seasons (PHOTOS)

Jessica Napoli

The Big Bang Theory has begun its 12th and final season.

The CBS comedy series premiered September 24, 2007, and now, after over a decade on TV, the cast looks a little different than when the show first aired.

Where Does 'The Big Bang Theory' Rank Among the Longest-Running TV Sitcoms?

Where Does 'The Big Bang Theory' Rank Among the Longest-Running TV Sitcoms?

Breaking down American sitcoms with the highest episode counts, including 'Married… With Children,' 'Cheers,' and more.

From costumes to haircuts, the show's leads — Johnny Galecki, Jim Parsons, Kaley Cuoco, Simon Helberg, Kunal Nayyar, Mayim Bialik and Melissa Rauch — certainly have changed, but the laughs are just as fresh.

Click through the gallery above to see how each actor grew up right before our eyes.

The Big Bang Theory, Thursdays, 8/7c, CBS