'NCIS' Celebrates 350 Episodes — See How the Cast Has Changed (PHOTOS)

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THEN (2004): Mark Harmon

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NOW (2017): Mark Harmon

Actor David McCallum Attends Juror Appreciation Day
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THEN (2002): David McCallum

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NOW (2017): David McCallum

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THEN (2003): Pauley Perrette

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NOW (2017): Pauley Perrette

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THEN (2004): Michael Weatherly

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NOW (2016): Michael Weatherly

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THEN (2003): Cote de Pablo

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NOW (2015): Cote de Pablo

Navy NCIS: Naval Criminal Investigative Service Celebrates Their 100th Episode
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THEN (2007): Sean Murray

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NOW (2017): Sean Murray

20 Annual Casting Society Of America Artios Awards
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THEN (2004): Rocky Carroll

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NOW (2017): Rocky Carroll

27th Annual PaleyFest Presents
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THEN (2010): Brian Dietzen

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NOW (2017): Brian Dietzen

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It's the end of an era for NCIS fans, as Pauley Perrette prepares to leave the long-running CBS series. But before that, the CBS series has something good to celebrate: 350 episodes!

On April 17, the long-running procedural hits the major milestone, reminding us just how much the show and its cast have changed through the years. In fact, Perette isn't the first to make her exit — in the show's earlier years, Mark Harmon was flanked by the likes of Michael Weatherly (Bull) and Cote de Pablo, both who have since left.

Here's What We Know About Pauley Perrette's Final 'NCIS' Episodes as AbbySee Also

Here's What We Know About Pauley Perrette's Final 'NCIS' Episodes as Abby

The actress has appeared in over 350 episodes since the 2003 premiere.

Still, a number of favorites are still around, such as Sean Murray and David McCallum. And in honor of the big episode, we're taking a look at the stars who made NCIS what it is today from way back when to now. Click through the gallery above for them all!

NCIS, 350th Episode airs Tonight, Tuesday, April 17, 8/7c, CBS