"Biology" is a song performed by English-Irish all-female pop group Girls Aloud, taken from their third studio album Chemistry (2005). The song was written by Miranda Cooper, Brian Higgins and Higgins' production team Xenomania, and produced by Higgins and Xenomania. Composed of distinct sections, it avoids the verse-chorus form present in most contemporary pop music. "Biology" was released as a single in November 2005, ahead of the album's release. Following the disappointment of "Long Hot Summer", "Biology" returned Girls Aloud to the top five of the UK Singles Chart and became their tenth top ten hit.

The music video, consisting only of group shots, witnesses Girls Aloud seamlessly move through various sequences while performing disjointed choreography. "Biology" was promoted through a number of live appearances and has since been performed on all of Girls Aloud's subsequent concert tours. The song, which includes a variety of styles, received widespread acclaim from contemporary music critics. Considered one of Girls Aloud's signature songs, The Guardian referred to "Biology" as "the best pop single of the last decade."

"Biology" is composed of a number of distinctly different sections. The song begins with Nadine Coyle singing over a bluesy piano riff, based on the main riff from The Animals' "Club A-Gogo". The first verse occurs, followed by two noticeably individual transitional bridges. Around two minutes into the song, the song reaches its climactic chorus before returning to the stanza heard in the introduction. The song repeats the chorus and the introduction is also used as an outro. The song avoids the typical AABA form and verse-chorus form present in most contemporary pop music.

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