Nucleus was a British jazz-fusion band, which continued in different forms from 1969 to 1989. In 1970, the band won first prize at the Montreux Jazz Festival, released the album Elastic Rock, and performed both at the Newport Jazz Festival and the Village Gate jazz club.
The band was established by Ian Carr, who had been in the Rendell-Carr Quintet during the middle and late 1960s. Nucleus's debut album, Elastic Rock, and the next two collections, We'll Talk About It Later and Solar Plexus, were all released on Vertigo Records, and music journalist Colin Larkin noted were "vital in any comprehensive rock or jazz collection".
In August 2005, a reincarnation of Nucleus with old and new members performed at Cargo in London. This was followed on 30 March 2007 by a Nucleus Revisited concert at London's PizzaExpress Jazz Club as part of a series of concerts to mark the tenth anniversary of Jazzwise magazine. Nucleus Revisited included Geoff Castle, Mark Wood, and Tim Whitehead and on trumpet, as at the 2005 Cargo concert, Chris Batchelor. Although Ian Carr did not play due to ill health, he was present at the concert and received a standing ovation.