"Jealousy" is a song originally written in 1982 by the Pet Shop Boys, recorded for their 1990 album Behaviour. In 1991, it was released in a slightly remixed form as a single, which appears on both Pet Shop Boys' greatest hits albums. It has also been covered by the British band Dubstar, and was sung by Robbie Williams at the 2006 Pet Shop Boys' BBC Radio 2 concert at the Mermaid Theatre, a recording of which was released on the Pet Shop Boys' live album Concrete.

In the Further Listening 1990–1991 booklet (enclosed with the 2001 2-CD re-release of Behaviour), Neil Tennant states that "Jealousy" is the first proper song ever composed by the duo. Chris Lowe composed the melody at the piano in his parents' home and, as he felt it should be a ballad, asked Tennant to write an intense-sounding lyric. Tennant complied by writing a lyric about the simplest form of jealousy: infidelity suspicions aroused by someone's indifferent or disrespectful attitudes towards another person's feelings (such as making his/her partner wait all night for a phone call which never comes). The song was then left off four albums because the duo were waiting for legendary film composer Ennio Morricone to agree to score the orchestral arrangement for the song. Morricone's answer never came, and Harold Faltermeyer ended up doing the arrangement for the song's release on Behaviour.

The album version, coming at the end of Behaviour, closed off the album with a sampler-based orchestral outro. The single version is slightly remixed, and uses a real orchestra instead during the outro. The extended version of the single version lengthens the outro while adding an orchestral intro as well; in addition, Neil Tennant recites a quote from William Shakespeare's Othello over both sequences:

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