"Who Is It? (Carry My Joy on the Left, Carry My Pain on the Right)" commonly abbreviated as "Who Is It?" is a song recorded by Icelandic singer Björk for her 2004 studio album Medúlla. It was released as the first single from the album on 18 October 2004, by One Little Indian Records.

The accompanying music video for the song was directed by Dawn Shadforth and shot outdoors in Björk's native Iceland. The video features the bell choir version of the song, which was re-recorded with the Bústaðakirkja Bell Choir, and is different from the album version.

The first version of "Who Is It" was written with Bogdan Raczynski. He stated that Björk approached him during his show in London in 2000. "We literally bumped into each other while I was playing onstage, doing vertical jumps, trying to smash the tables and maximum raving - and apparently she was too", he said. Months later, they met in New York and wrote a song titled "Embrace Fortress" during the final recording sessions for her fourth studio album Vespertine (2001). She decided not to put the song on the album because she felt it was from a different family; according to Björk, Vespertine was "introvert and shy and not a very physical record", and she felt this was a physical song that she wrote when she was feeling quite strong again. Raczynski also said he did not want his version of the song to be available. He said "It was a beautiful thing we did together, but for that reason alone it should be left in the clouds. People need mystery and romance". However, it eventually received a limited vinyl release by Rephlex Records.

Björk then approached the song again during the recording sessions for Medúlla and was renamed "Who Is It". She explained that it was the one song that Matmos helped her with – they did "fruit-machine-noises" that are in the choruses. He made the noises with synthesizers that she later imitated with her voice. It was also the first song that Rahzel sang, doing the beat in one take. The singer said that after he recorded, "we just all fell on the floor, we couldn't believe it. There's no overdubs, no treatment – this is it. I felt quite proud – if you're gonna have a human beatboxer, at least get the real thing".

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