Fantasia No. 3 in D minor, K. 397 (Fantasy in English, Fantasie in German) is a piece of music for solo piano composed by Wolfgang Amadeus Mozart in 1782. Despite being unfinished at Mozart's death, the piece is nonetheless one of his more popular compositions for the piano. Because of its somewhat unusual rhythm, its constantly changing tempo (seven different tempi occur throughout the piece, some of which are quite fast, including the three meter-less cadenzas), and its complete lack of any recognizable musical form (as indicated by the "Fantasy" title), the Fantasia is considered to be a relatively challenging piece to perform.
The original manuscript has not survived and the final measures of the piece have been lost. The ending as it currently exists is believed to have been written by August Eberhard Müller, one of the composer's admirers.
The Fantasia runs to just over 100 measures, and a full performance takes approximately seven minutes.
^ Hirsch, Paul. “A Mozart Problem.” Music and Letters 25, 4 (1944): 209–12.