Eminem is headlining Chicago's Lollapalooza Festival and closed out his day one performance with a sample of things to come. After running through his slew of hits, Shady brought out Rihanna for their smash "Monster." Ri Ri also assisted Em with the performance of his classic track, "Stan." In a few days, the two mega stars will embark on their The Monster tour which is set to kick off at the Rose Bowl in Los Angeles on August 7.

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