The final version features throat singer Tanya Tagaq and beatboxer Rahzel, and lyrics reflecting the dialogue between mother and child.

The music video was directed by Dawn Shadforth and shot outdoors in Iceland, and features Björk in a bell-shaped dress designed by Alexander McQueen.

During the promotional campaign for Medúlla, Björk performed the song on a number of occasions, such as Friday Night with Jonathan Ross and L'Album de la semaine.

Björk approached him during a show in London in 2000, and 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).

[5] The echo effects in "Who Is It" may additionally reflect the "scattered sense of self the mother may experience as she carries the burden of constant care for her child".

[7] Pitchfork's Dominique Leone said "Who Is It" reminded him of "Alarm Call" from Homogenic (1997), in the way it "applies Björk's idiosyncratic performances to a traditionally pleasant sounding template", though he felt 'Who Is It' featured a "much more interesting" chord progression during the verses, and an "altogether incredible rhythm track".

[9] Melanie Haupt commented that Björk's "trills in her native Icelandic work well on "Who Is It," which underscores this disc's experiment in taking the raw power of the human voice".

[12] 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".

[13] 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".

[14] 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".

[15] 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)".

[16] 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".

[23][24] The video is divided into scenes in the day and night in a desert, with Björk wearing the bell-shaped dress designed by McQueen,[25] dancing playfully to the song.

[28][29] During the promotional campaign for Medúlla, a bell choir mix of "Who Is It" was performed with Rahzel and an English bell choir at BBC's Friday Night with Jonathan Ross on 8 October 2004,[30] and at Maida Vale Studios during a set of songs for Gilles Peterson's radio show on BBC Radio 1, two days later.