The Piano Sonata No. 14 in C♯ minor "Quasi una fantasia", op. 27, No. 2, by Ludwig van Beethoven, popularly known as the Moonlight Sonata (Mondscheinsonate in German), was completed in 1801. It is rumored to be dedicated to his pupil, 17-year-old[2] Countess Giulietta Guicciardi, with whom Beethoven was, or had been, in love.[4] This was one of Beethoven's most popular sonatas.

  The name "Moonlight" Sonata derives from an 1832 description of the first movement by music critic Ludwig Rellstab, who compared it to moonlight shining upon Lake Lucerne.

  Beethoven included the phrase "Quasi una fantasia" (Italian: Almost a fantasy) in the title partly because the sonata does not follow the traditional movement arrangement of fast-slow-fast. Instead, the Moonlight sonata possesses an end-weighted trajectory; with the rapid music held off until the third movement. To be sure, the deviation from traditional sonata form is intentional. In his analysis of the Moonlight sonata, German critic Paul Bekker states that “The opening sonata-allegro movement gave the work a definite character from the beginning... which succeeding movements could supplement but not change. Beethoven rebelled against this determinative quality in the first movement. He wanted a prelude, an introduction, not a proposition.”