Frédéric Chopin's Fantaisie-Impromptu (Polish: Fantazja-Impromptu) in C♯ minor, Op. posth. 66, is a solo piano composition. It was composed in 1834 and published posthumously in 1855 despite Chopin's instruction that none of his unpublished manuscripts be published. The Fantaisie-Impromptu is one of Chopin's most frequently performed and popular compositions.