The Insider.com

Paris Hilton and Nicole Richie

Paris and Nicole have been in an on-again, off-again friendship for years, knowing each other since childhood. Despite rumors, neither Paris or Nicole have revealed what caused their rift dating back to 2005. According to People.com, in April 2005, Paris said in a statement, "Nicole knows what she did, and that's all I'm ever going to say about it."