'Big Bang Theory': Bob Newhart, Regina King & More Bangin' Guest Stars (PHOTOS)

Jim Halterman

The Big Bang Theory has had many memorable guest stars over its 12 seasons, but some were really out of this world.

Ahead of the CBS comedy's finale on May 16, the cast has been looking back on the series' beginnings, fandom, catchphrases, and more. And what would TBBT be without the colorful characters (and familiar faces) that stopped by through the years?

Click through the gallery above for our picks of the most iconic guest appearances, from Bob Newhart to Regina King, and hear what the actors had to say about their characters!

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

Johnny Galecki & Kaley Cuoco Get Silly to Celebrate the 'Big Bang Theory's Final Episodes (PHOTOS)

Johnny Galecki & Kaley Cuoco Get Silly to Celebrate the 'Big Bang Theory's Final Episodes (PHOTOS)

The co-stars and on-screen spouses had fun ahead of the show's big farewell.