No Doubt lead singer Gwen Stefani wrote this with Dave Stewart of Eurythmics fame. They were introduced by Jimmy Iovine, who is good friends with Stewart and chairman of Interscope, which is No Doubt's label. The band came over to Stewart's apartment in London, and he and Gwen went to the kitchen with their acoustic guitars and completed the song in about 15 minutes. Stewart wrote of Stefani in The Dave Stewart Songbook: "She really has great melodic sensibility and uses her subconscious to express herself lyrically. She sings freeform stream-of-consciousness at first but there are always words that appear to be coming from an emotional place. She then quickly understands what she wants to say and it all falls into place. 'Underneath It All' was an example of this and I saw it happen in real time right there in my kitchen."

In our interview with Dave Stewart, he spoke about the difference between writing with his Eurythmics partner Annie Lennox, and writing with Stefani, who he had just met. Says Stewart: "Well, it's kind of weird. Because if I walk into a room to write with Gwen, Gwen comes with a lot of stuff attached to her, and I also come into a room with that. So the first 15 minutes usually is getting past that stuff. And then very quickly, and usually due to a certain courtship, you know like how a certain chord can change the temperature in a room? It evokes a certain emotion in me or in Gwen, and then it'll suddenly make us think about something that happened in her life, and something that she might want to talk about, something she might not want to talk about, but you can kind of hear what it is in the lyric and the melody. And soon, within an hour and a half it's as if you've known that person for years. And often, then, people start talking about all their private thoughts, and everything that's going on in their life usually comes rolling out."

In a 2002 interview in Rolling Stone, Stefani explained that her lyrics were inspired by her relationship with Gavin Rossdale of the band Bush, who she was madly in love with and later married. They spent some time in a part the previous day, and Stefani wrote in her journal: "You're lovely underneath it all."

Gwen Stefani and Dave Stewart hadn't met before the day they wrote this song, but they had a lot in common and bonded quickly. They both had intimate relationships with a band member who they broke up with, but continued working with: Stewart with Annie Lennox and Stefani with No Doubt bass player Tony Kanal. They share a love of Jamaican music, and are big fans of Sophie Muller, who has directed videos for Eurythmics and No Doubt.

This track features a rap by the Jamaican Dancehall deejay Lady Saw. In 2003 she became the first female deejay to win a Grammy when this won the Grammy for Best Performance by a Duo or Group with a Vocal.