"Lost!" is a song by the British alternative rock band Coldplay. The band co-produced it with Brian Eno and Markus Dravs for their fourth album, Viva la Vida or Death and All His Friends. The song was released on 10 November 2008 as the fourth overall single from the album to generally positive critical reviews. A live version was released via digital download following a performance of the band and Jay-Z at the 2009 Grammy Awards, spurring high digital sales and giving "Lost!" a new peak at number 40 in the United States.

Several official versions of the song exist, including vocalist Chris Martin's separate recording with a piano accompaniment (known as "Lost?") which appears as a B-side for "Violet Hill", and a version that features Jay-Z (known as "Lost+") which appears on their 2008 EP Prospekt's March. The accompanying music video to the single features a live performance of the band in the United States. Coldplay launched a contest through their website, in which fans submitted self-made music videos. The winning entries were posted on the band's website. The song was used in the 2010 film, The Way.

According to Coldplay drummer Will Champion in a fan mailout, "Lost!" had been developed long before the band began recording tracks for their fourth studio album, Viva la Vida or Death and All His Friends, and was not getting into completion. Champion recalls, "I was starting to get into using a particular drum programming programme. I was messing around with that and I built this beat up from scratch. ... Then Chris overheard it and played 'Lost!' over the top of it." Martin revealed that the song's drumbeat took inspiration from Justin Timberlake's "Cry Me a River", which was one of Champion's favourite songs at that time.

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