Spence D. of IGN Music said that despite the "strangely haunting" intro, the song was the most "straight forward, accessible number" on the album. Jennifer Vineyard from MTV News shared a similar sentiment, stating that despite its "cumbersome title", it was the "most immediate and catchy number, with a pure sense of seize-the-day joy: If there is to be a single released from the album, this should be it". "The Guardian"s Michael Cragg stated that its verses "melt into a big, melodious chorus that showed that despite her disappearance from daytime radio, Björk still knew how to make something resembling a crowd singalong". Matthew Gasteier of "Prefix" magazine was also positive, calling the song a "mostly typical Bjork anthem", replacing her previous electronic sound with the "concept-appropriate but still conventional human beat". AllMusic's Heather Phares noted that the song, along with promotional single "Oceania", "have an alien quality that is all the stranger considering that nearly all of their source material is human (except for the odd keyboard or two)". Jon Pareles from "The New York Times" stated that, along with "Where Is the Line", "Who Is It" is a "wide open [song] with complex superstructures, catchy but ghostly, and they probably haven't seen their final incarnation". BBC Music's David Hooper called the song "enjoyable", citing it as one of the only "really immediate tunes" on "Medúlla". In the United Kingdom, "Who Is It" debuted at its peak of number 26 on the UK Singles Chart on the issue dated 24 October 2004. It became her highest peaking single since "Hidden Place", which reached number 21 in 2001. The song also obtained the same position in Italy, on the issue dated 2 December 2004, but fell to number 40 the next week, before falling off the chart. In France, "Who Is It" debuted at its peak of number 62, before spending two other weeks and falling off the chart. The accompanying music video for "Who Is It" was shot in early September 2004 in , Iceland and directed by Dawn Shadforth. It premiered at the grand opening of the "Islande de glace et de feu" event at the Palais de la Découverte, in Paris, France, on 27 September 2004. For the video, Björk approached her friend and collaborator Alexander McQueen to design a dress she would wear for it.