1.Zac Efron posted this photo of when he hung out with Leo DiCaprio at a Lakers' game in the '00s.
2.Paris Hilton shared some photos of when she and Kim Kardashian took a girls trip to Ibiza in 2006.
3.Mandy Moore shared a photo of when she opened for NSNYC in 1999.
4.Danielle Fishel posted this oh-so-'90s photo of herself alongside her Boy Meets World co-star, Rider Strong.
5.Topher Grace shared this pic of himself and Ashton Kutcher decked out in late-'90s Abercrombie & Fitch.
6.Orlando Bloom posted a whole collage of photos of himself throughout the years.
7.Marisa Tomei took it back to her A Different World days and shared this photo of herself and her BFF/co-star, Lisa Bonet.
8.Will Smith made his adorable baby photo his very first #TBT.
9.Beyoncé celebrated the 15th anniversary of her and Jay-Z's classic duet "'03 Bonnie & Clyde," by posting this clip from the music video, and also a clip of the two of them promoting it on TRL.
10.Elizabeth Banks shared this photo of herself at 14 years old.
11.Katy Perry posted this too cute childhood photo of herself in anticipation of her Witness: The Tour.
12.Freddie Prinze Jr. shared this photo of himself alongside Anna Paquin and Rachael Leigh Cook at the premiere of their classic film, She's All That, in 1999.
13.Mark Ruffalo shared this video clip from when he appeared in a zit commercial in the '80s.
14.Lupita Nyong'o posted this photo of herself in 2001.
15.Christina Aguilera celebrated the 15th anniversary of her iconic album Stripped by sharing photos from the album linear notes.
16.Madonna remembered her ex-boyfriend, the late Tupac Shakur.
17.Dan Levy posted this cute childhood photo of himself with his dad, the great Eugene Levy.
18.Sarah Michelle Gellar celebrated the 20th anniversary Scream 2, by sharing this photo of herself alongside her co-stars Jada Pinkett Smith, Neve Campbell, and Courteney Cox.
19.Salma Hayek remembered when she took superstar Mexican singer Luis Miguel to the Oscars in 1997.
20.Kim Kardashian shared this cute childhood photo of herself and big sister Kourtney wearing matching outfits.
21.Tyler Oakley posted this photo of himself on Christmas morning in 1996.
22.Hugh Jackman shared his track team school photo.
23.Britney Spears served up pure early-'00s fashion with this #TBT.
24.Selena Gomez posted a Wizards of Waverly Place then and now cast photo, in honor of her co-star David Henrie's wedding.
25.Janet Jackson shared this GIF of her and Michael Jackson presenting at an award show in the '70s.
26.Tracee Ellis Ross posted this photo of when she did some test modeling in high school in the late '80s.
27.Stephen Colbert shared this awkward puberty photo as part of the #PuberMe campaign, which helped raise money for Puerto Rico.
28.Alison Brie posted this photo as part of the #PuberMe campaign.
29.Mark Hamill shared this heartwarming photo of him and Carrie Fisher dancing.
30.Chris Hemsworth shared this childhood photo of himself camping with his mom.
31.Joseph Gordon-Levitt posted this photo of him and Zooey Deschanel hanging at Sundance in the early '00s.
32.Ewan McGregor posted this cheeky pic of himself and his BFF (and former roommate) Jude Law from the early '00s.
33.Justin Theroux shared this photo of himself at 14 years old.
34.Diana Ross posted this classic photo of herself from the '70s, as her very first Instagram photo.
35.Barbra Streisand remembered her iconic 1963 appearance on The Judy Garland Show.
36.Ariana Grande posted this sweet photo of her dad holding her as a baby.
37.Paul McCartney posted this photo of him and his fellow Beatles in honor of the 50th anniversary of their groundbreaking album Sgt. Pepper's Lonely Hearts Club Band.
38.In honor of the 20th anniversary of her death, Elton John shared this photo of himself alongside his good friend, the late Princess Diana.
39.Gal Gadot remembered her Fast and Furious days.
40.Naomi Campbell shared a behind-the-scenes clip from the iconic music video for George Michael's "Freedom."
41.Chris Pratt posted this behind-the-scenes Parks and Rec photo of himself and Audrey Plaza.
42.Dolly Parton shared this photo of herself, Rod Stewart, and Cher hanging out together in 1978, during the filming of a Cher TV special.
43.Seth Rogen posted this behind-the-scenes photo of himself and James Franco on the set of Freaks and Geeks.
44.Lin-Manuel Miranda posted this cute video clip of himself in the '80s.
45.And finally, Rashida Jones had the most EPIC photo of herself hanging out with her dad Quincy Jones, her siblings Kidada and Quincy Jones III, Paul and Linda McCartney, and their kids Stella and James, and Michael Jackson.