1.Friends co-stars Jennifer Aniston, Lisa Kudrow, and Matthew Perry all appeared in episodes of Courteney Cox's series Cougar Town.
2.The Office co-stars, and BFFs IRL, Mindy Kaling and B.J. Novak reunited on-screen in The Mindy Project when Novak appeared as Jamie, one of Mindy Lahiri's love interests.
3.Buffy the Vampire Slayer co-stars Alyson Hannigan and Seth Green reunited on How I Met Your Mother when Green played an annoying college classmate of Lily and Marshall's.
4.Drake & Josh co-stars Drake Bell and Josh Peck reunited on Grandfathered when Bell played an investor interested in Gerald's app.
5.Firefly stars Nathan Fillion and Gina Torres reunited on Castle when Torres played a murder suspect in Season 5.
6.Beverly Hills, 90210 co-stars Jennie Garth and Luke Perry reunited when Perry guest-starred on What I Like About You as a guy Val hooked up with in high school.
7.Parenthood mother-daughter duo Lauren Graham and Mae Whitman reunited on Gilmore Girls: A Year In The Life when Whitman played a girl Rory interviews for a story.
8.Seinfeld stars Jerry Seinfeld, Julia Louis-Dreyfus, Michael Richards, and Jason Alexander reunited with creator Larry David on Curb Your Enthusiasm when they played themselves.
9.Glee co-stars Darren Criss, Grant Gustin, and Melissa Benoist reunited when Criss guest-starred on a crossover episode of The Flash and Supergirl as the Music Meister.
10.Mad Men stars January Jones and Jon Hamm reunited on The Last Man on Earth when Hamm appeared as Darrell, a friendly intruder Melissa accidentally shoots.
11.The West Wing co-stars Allison Janney and Bradley Whitford reunited when Whitford guest-starred on Mom as a friend of Adam's.
12.8 Simple Rules mother-daughter duo Katey Sagal and Kaley Cuoco reunited when Sagal played Penny's mother on The Big Bang Theory.
13.Dawson's Creek stars Busy Philipps and James Van Der Beek reunited on Don't Trust the B---- in Apartment 23 when Philipps appeared as herself.
14.Glee co-stars Matthew Morrison and Sarah Drew reunited when Morrison guest-starred on Grey's Anatomy as Jo Wilson's abusive ex-husband.
15.Sister, Sister co-stars Tahj Mowry and Jackée Harry reunited when Harry appeared as a judge in several episodes of Baby Daddy.
16.Everybody Loves Raymond stars Patricia Heaton and Ray Romano reunited on The Middle when Romano played a guy who interrupts Mike and Frankie's wedding night.
17.Everwood father-daughter duo Emily VanCamp and Tom Amandes reunited when Amandes appeared on Revenge as Emily Thorne's lawyer.
18.Friday Night Lights stars Scott Porter and Matt Lauria reunited when Porter played Amber's boyfriend in the series finale of Parenthood.
19.The Mary Tyler Moore Show co-stars Betty White, Mary Tyler Moore, Cloris Leachman, Georgia Engel, and Valerie Harper reunited on Hot In Cleveland when Elka and Mamie reunited their old bowling team.
Who are your favorite TV co-stars that have reunited on other shows? Tell us in the comments below!