Transcendental Étude No. 5 in B-flat "Feux follets" (Will o' the Wisp) is the fifth etude of the set of twelve Transcendental Etudes by Franz Liszt.
As with the other works in the Études, Feux follets went through three versions, the first being Étude en douze exercises from 1826, the second being Douze études d'execution transcendentale from 1838, and third, an 1851 revision of the 1838 set. It is this last version, from 1851, that is most often performed, most of the demands of the 1838 version being trimmed down and refined. Its rapid double-note passages in the right hand accompanied by wide broken intervals in the left are especially notorious and difficult to play. It reaches several climaxes that are technically demanding and ends in pianissimo arpeggios. Despite the mechanical difficulties of the work, its greatest challenge lies in doing justice to its whimsical and mysterious character. Pianissimo and leggierissimo markings abound in the double note sections making it more difficult to play. It is one of the most technically difficult pieces of the whole set.