The best thing about going to a high school reunion, is seeing how everyone’s lives -- and looks -- have drastically changed!

When American Pie hit movie theatres in 1999, the cast of young actors was thrust into the public eye becoming international stars overnight.

Real Housewives of Beverly Hills star Kim Richards checked in a rehab facility this week, joining a slew of other Hollywood stars who have dealt with their demons in a treatment center.

RadarOnline.com has photos of all the biggest names in the business, from chart-topping singers to Academy Award winning actors, who have admitted to undergoing rehab programs to deal with their problems.

PHOTOS: Hollywood Stars Who Have Been To Rehab

What do Charlie Sheen, Robert Downey Jr. and Demi Moore have in common? All three Brat Packers have attended treatment programs. While Demi only went to rehab once in the 1980's to deal with a cocaine problem, Sheen and Downey have done the nationwide tour, attended several different programs over the years.

Universal managed to wrangle the cast of American Pie together for a reunion movie more than 10 years after the original was made, but you’ll be surprised to learn what each cast member is earning!

RadarOnline.com has learned the $50 million film doled out salaries to stars ranging from $250,000 all the way up to $5 million.

According to the Hollywood Reporter, Seann William Scott and Jason Biggs are earning a whopping $5 million apiece while Tara Reid is the least paid member of the cast, raking in a comparatively paltry, $250,000.

Actor Seann William Scott -- best known for his roles as Stifler in the American Pie movies and most recently for his appearance in the Bruce Willis film Cop Out -- has entered rehab, RadarOnline.com has learned.

A rep for Scott, 34, released this brief statement: "Seann William Scott has voluntarily admitted himself for proactive treatment to address health and personal issues. He appreciates the support of his many fans at this time."

Scott's camp did not release the exact reason for him entering rehab, nor the name or location of the facility where he's being treated.