Eminem headlined Lollapalooza tonight, and he surprised the audience by bringing out Rihanna. The two performed "Monster," "Love The Way You Lie," and "Stan" together. This serves as a preview for the upcoming Monster tour which will begin next week at the Rose Bowl. Aside from the special guest, Em's set in Chicago included a mix of his classics  and songs off his latest album, The Marshall Mathers LP 2, such as "Rap God" and "Berzerk."