When Ashlee Simpson was caught lip-syncing on Saturday Night Live on October 23rd, it was an embarrassment for the singer and the show. But the use of backing tracks in concerts and TV performances is hardly the shock it was when Milli Vanilli was revealed to have lip-synced in 1990.

"In the long run, it's about performance," says Alexandre Magno, a choreographer on Madonna's 1993 Girlie Show Tour. Madonna did most of the vocals live, he says, but some were prerecorded to play back during complex dance numbers. "If you're up there and you know what you're doing, it doesn't matter if you're lip-syncing or you're singing live," says Magno.

"It's extremely common," says Simpson, who maintains she was unable to sing because she was ill with acid reflux. "You go to people's tours, and you see them dancing extremely hard -- do you really think they're singing that well all the time? You try it. You'd be gasping for breath. At awards shows they're singing so perfect, and it's definitely not just them."

Reviews of Britney Spears throughout her career often mention lip-syncing. Spears has denied the allegations, although earlier this year her then manager, Larry Rudolph, told the New York Times that she occasionally lip-syncs while dancing.