Frédéric Chopin’s Fantaisie-Impromptu in C-sharp minor, Op. posth. 66, is a solo piano composition. The Fantaisie-Impromptu was written in 1834, as were the Four Mazurkas (Op. 17), and the Grande valse brillante in E-flat major (Op. 18), but, unlike these other works, Chopin never published the Fantaisie-Impromptu. Fantaisie-Impromptu draws many of its harmonic and tonal elements from Beethoven’s Moonlight Sonata, which is also in C-sharp minor, in particular, the third movement.