Fun (often stylized as fun.) is an American alternative rock band based in New York City that was formed by Nate Ruess, formerly of The Format. After the 2008 breakup of The Format, Ruess formed Fun with Andrew Dost and Jack Antonoff of Anathallo and Steel Train respectively. Fun has released two albums: their debut Aim and Ignite in 2009 and their latest Some Nights in February 2012.

The band is best known for its hit single "We Are Young" featuring Janelle Monáe. "We Are Young" reached number one on the U.S. Billboard Hot 100 and Digital Songs charts, becoming the first alternative song to top the Billboard Hot 100 chart since Coldplay's "Viva la Vida" in 2008. It also peaked at number one on the UK Singles Chart.

The Format split in February 2008. Immediately afterward Nate Ruess asked Andrew Dost and Jack Antonoff to join his new project. Dost had toured with The Format and provided various instrumentation. Ruess met Antonoff after The Format toured with Steel Train. The three began working together in New Jersey within a week. Ruess sang melodies while the other two provided music for them. The first demo song the band recorded was "Benson Hedges," which was made available for free in Spin's September 20, 2008 article on the band. Fun approached Steven McDonald, who produced The Format's album Dog Problems with Ruess, to produce their debut album. McDonald was enthusiastic about the project and stated, "I can’t believe what we’re working on here. This crushes anything I’ve ever done."