PHOTOS: MASSIVE Celebs Strip Down for "Towel Series" Photoshoot

All the biggest stars are taking it all off for Mario Testino!

Well...almost all, everything except a towel.

Justin Bieber, Keira Knightley, Kendall Jenner and more feature in the latest installment of Testino's "Towel Series."

The renowned photographer debuted the photo shoot in V Magazine's 100th issue, which features Britney Spears as its cover girl.