Taylor Swift joined the Rolling Stones to lend her voice on "As Tears Go By" during the last night of the Stones' three-day stint in Chicago at the United Center.

This is actually the first song that Keith Richards and Mick Jagger wrote together.

The Stones announced Swift's appearance on Twitter Monday night. The group is currently celebrating their 50th anniversary with the 50 & Counting Tour, which has featured guest spots from artists from so many different genres, including a few appearances from country music stars.

Keith Urban joined the group onstage in Los Angeles on May 3, Carrie Underwood came out in Toronto to sing on "It's Only Rock 'n' Roll (But I Like It)", Sheryl Crow was a guest and now Taylor Swift.

Pretty cool to be 23 and jamming out the with the Rolling Stones!