Get ready for a diverse cast of performers at this year's Grammy Awards, from today's favorite pop stars to rock icons of generations past. Sixteen-year-old best new artist nominee Justin Bieber kicked off rehearsals with a medley that began on his acoustic guitar then segued into an amped-up version of his new song, "Never Say Never," backed by fire-breathers. Usher, who has won five Grammys and is up for two more awards on Sunday, followed Bieber with his dance track "OMG." Later, 69-year-old rocker and 10-time Grammy winner Bob Dylan belted out his 1965 blues track "Maggie's Farm" with a chorus of strings in the background. Dr. Dre, Rihanna, Eminem, Katy Perry, Lady Gaga, Barbra Streisand, and Mick Jagger will be among the other performers on Sunday night's show.