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

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CBS
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Robert Voets/CBS

Sheldon Cooper

Played by: Jim Parsons (Season 1)

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Michael Yarish/Warner Bros.

Sheldon Cooper

Played by: Jim Parsons (Season 12)

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Robert Voets/CBS

Penny

Played by: Kaley Cuoco (Season 1)

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Michael Yarish/Warner Bros.

Penny

Played by: Kaley Cuoco (Season 12)

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Robert Voets/CBS

Leonard Hofstadter

Played back: Johnny Galecki (Season 1)

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Michael Yarish/Warner Bros.

Leonard Hofstadter

Played by: Johnny Galecki (Season 12)

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Mike Ansell/Warner Bros.

Howard Wolowitz

Played by: Simon Helberg (Season 1)

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Michael Yarish/Warner Bros.

Howard Wolowitz

Played by: Simon Helberg (Season 12)

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CBS

Rajesh Koothrappali

Played by: Kunal Nayyar (Season 1)

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Michael Yarish/Warner Bros.

Rajesh Koothrappali

Played by: Kunal Nayyar (Season 12)

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Sonja Flemming/CBS

Amy Farrah Fowler

Played by: Mayim Bialik (Season 3)

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Michael Yarish/Warner Bros.

Amy Farrah Fowler

Played by: Mayim Bialik (Season 12)

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Cliff Lipson/CBS

Bernadette Rostenkowski-Wolowitz

Played by: Melissa Rauch (Season 3)

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Michael Yarish/Warner Bros.

Bernadette Rostenkowski-Wolowitz

Played by: Melissa Rauch (Season 12)

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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.

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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