Prepon, 38, paid tribute to the show with a group pic from the gang's first press tour. In the shot, Mila Kunis, Ashton Kutcher and Valderrama join Prepon on an incredible, open air bus that's decorated with an ad for the show.
"#ThrowbackThursday to the first press tour for #That70sShow - we were all blown away when we saw this bus! Can’t believe the show came out 20 years ago today…such a special group of people," the Orange Is the New Black star captioned the pic. "So lucky to have been a part of it. ❤️❤️❤️ I think Hyde is taking the pic! #tbt."
Valderrama honored the special occasion with a slideshow of shots from his time on the show.
"August 23, 1998.. this was the first picture we ever took together, 20 years ago today... We had just met and had no idea what was ahead of us, or if it would even last.. but we knew it was gonna be fun," Valderrama, 38, began a lengthy post. "We spent 8 years and 200 episodes together, we traveled the world... we grew up together. We were a family, and will always be."
The NCIS actor went on to thank the directors, creators, producers, writers and crew of the show for "believing in these young hooligans," before shifting his gratitude to the fans.
"To our beautiful fans who followed and went on this wild ride with us, THANK YOU, you changed our lives," he gushed.
Valderrama concluded his note with funny messages to each of his co-stars.
"To my brothers and sisters... you know... umm the cast.. like uuuh.. the kid who had the two scenes in Traffic, the cat with the curly hair that was in Dracula 2000, that Russian girl who said she was 18 but was actually 14 in the pilot.. the chick on Orange Is the New Black, the guy who hosted Punk’d and the bad ass who played the main villain on Robocop.. the Italian guy who did a great Tom Jones, our very own Bond Girl, our Chong Smoke Machine and of course Debra Jo Rupp who played Kitty (who can forget her)," he quipped. "I love you guys always, you guys made me who I am today.. and we will always have each other..#That70sShow #20Years #TBT."
Meanwhile, 40-year-old Grace shared a snap of the cast making silly faces for the camera.
"Ultimate #tbt - Hard to believe That 70s Show premiered 20 years ago today. It’s rare someone can say they owe everything to one small group of people, but in my case it’s true," Grace captioned the pic. "Too hard to watch now because I get so nostalgic, but I don’t need reruns to remember all of the amazing experiences and friendships. To quote the ‘78 Point Place High yearbook 'What a long strange trip it’s been... in Forman’s basement' #hellowisconsin #wereallalright #eric&donna&hyde&jackie&kelso&fezforever."
Grace recently spoke about the possibility of a That '70s Show reunion, revealing that he'd definitely be up for it.
"I always say I would love...I mean, absolutely love to do it," he told ET earlier this month. "I doubt it only because that cast went on to have so much success," he said. "If they're down, I'm down because I love those guys."