Frédéric Chopin's Fantaisie-Impromptu in C-sharp minor (Opus 66), is a solo piano composition and one of his best-known pieces. It was composed in 1834 and dedicated to Julian Fontana, who published the piece in spite of Chopin's request not to do so.

In the original pioan version, the piece uses many cross-rhythms (the right hand plays sixteenth notes against the left hand playing triplets) and a ceaselessly moving note figuration and is in cut time (2/2). It concludes in an ambiguous fantasy-like ending, in a quiet and mysterious way, playing the first few notes of the moderato section theme, while continuing with the sixteenth notes (semiquavers). The piece resolves and gently ends on a C-sharp major rolled chord.

Although originally composed for solo Piano, I adapted his work for flute and Concert (Pedal) Harp.