From Reese Witherspoon to Brad Pitt, Friends has had some very famous faces show up during the 10 years the show aired. But a few other famous celebrities who didn't star on Friends joined the cast for the two-hour reunion episode.
Reese Witherspoon made her first Friends appearance back in 2000 as Rachel's younger sister, Jill.
She appeared on the reunion, blushing over the fact she was able to hear Matt Le Blanc say Joeys iconic line "How you doin'".
The Academy Award-winning actress described the moment she was called in to play Rachel's spoilt sister on Friends as "winning the lottery".
The then-38-year-old actress only appeared on two episodes and was asked to appear on the show again but turned it down.
In an interview with Jennifer Aniston, Witherspoon told AP she was "too scared" to return to the iconic show.
"Did you know they asked me back and I said I can't do it? I got ... I was too scared.
"I got scared! They asked me to come back and I was like, 'I'm too nervous,'" she said.
Guest star Tom Selleck best known for his role as private investigator Thomas Magnum came into the Friends fictional world in the second season playing the role of Richard Burke, Monica's older boyfriend.
Selleck made a quick cameo during the first half of the reunion while the main characters were playing a quiz game. He was the mystery voice on a voicemail where the cast members had to figure out who he was.
Lisa Kudrow takes her position on the Central Perk couch to perform a rendition of Smelly Cat with a very special guest.
Sitting down next to the actress, dressed in a striking pink and orange outfit Phoebe would have rocked, Gaga belts out the verses with her, with a gospel choir bursting through the door for backup vocals.
"I still think it's better with just me," Kudrow jokes at the end.
Gaga went on to thank her for being "the different one" on the show.
Psst, here's a closer look at the 'fit:
Other surprise guests drop in
Towards the end of the episode, celebrities including Cindy Crawford and Justin Bieber walked the runway in infamous Friends costumes.
Cara Delevingne popped in modelling Rachel's ugly bridesmaid dress and also dressed as the Holiday Armadillo :
Cindy Crawford rocking Ross' tight leather pants:
And, bizarrely, Justin Bieber dressed in Ross' Spudnik potato costume:
Bieber's role in the reunion special in particular set tongues wagging on social media, with many wondering why he was chosen over former guests of the show such as Paul Rudd.
One famous face which seemed to stick around was Phoebe Buffay's boyfriend turned husband Paul Rudd. Who is now also known for his role in the Marvel films Ant-Man.
Rudd made his Friends debut as Phoebe Buffay's love interest on season nine.
Now 52, Rudd is married to wife Julie Yaeger.
But many fans were left wondering why random celebrities were chosen over Rudd.
- Additional reporting by news.com