"Rez" is a non-album track by British electronic music group Underworld, originally released in 1993 in the UK. It became a popular club hit and live track in Underworld's sets, despite the fact it could not be found on an album.
However, it did appear on many compilations, b-sides, and even appeared on a bonus disc for Second Toughest in the Infants (1996).
The first promo release of the single was pressed onto pink vinyl, and is highly collectible as it has an exclusive non-album track, "Why, Why, Why".
The song's obscurity led to many bootleg pressings of the song before it was officially re-issued as part of 1992–2012 The Anthology.
The more common release has the single "Cowgirl" as its b-side. This version also received a limited white vinyl pressing in the US.