2, is a piano sonata by Ludwig van Beethoven, completed in 1801 and dedicated in 1802 to his pupil Countess Julie "Giulietta" Guicciardi.

[3] "The subtitle reminds listeners that the piece, although technically a sonata, is suggestive of a free-flowing, improvised fantasia.

[5][6] This comes from the musicologist Wilhelm von Lenz, who wrote in 1852: "Rellstab compares this work to a boat, visiting, by moonlight, the remote parts of Lake Lucerne in Switzerland.

In fact Rellstab made his comment about the sonata's first movement in a story called Theodor that he published in 1824: "The lake reposes in twilit moon-shimmer [Mondenschimmer], muffled waves strike the dark shore; gloomy wooded mountains rise and close off the holy place from the world; ghostly swans glide with whispering rustles on the tide, and an Aeolian harp sends down mysterious tones of lovelorn yearning from the ruins.

[13] Many critics have objected to the subjective, romantic nature of the title "Moonlight", which has at times been called "a misleading approach to a movement with almost the character of a funeral march"[14] and "absurd".

[17] Gramophone founder Compton Mackenzie found the title "harmless", remarking that "it is silly for austere critics to work themselves up into a state of almost hysterical rage with poor Rellstab", and adding, "what these austere critics fail to grasp is that unless the general public had responded to the suggestion of moonlight in this music Rellstab's remark would long ago have been forgotten.

[22] Donald Francis Tovey warned players of this movement to avoid "taking [it] on a quaver standard like a slow 128".

The adagio sostenuto tempo has made a powerful impression on many listeners; for instance, Berlioz commented that it "is one of those poems that human language does not know how to qualify".

[23] Beethoven's student Carl Czerny called it "a nocturnal scene, in which a mournful ghostly voice sounds from the distance".

[1] The movement was very popular in Beethoven's day, to the point of exasperating the composer himself, who remarked to Czerny, "Surely I've written better things".

He claims to have found, in the archives of the Wiener Musikverein, a sketch in Beethoven's handwriting of a few lines of Mozart's music (which bears the same characteristic triplet figuration) transposed to C♯ minor, the key of the sonata.

[23] Beethoven's heavy use of sforzando (sfz) notes, together with just a few strategically located fortissimo (ff) passages, creates the sense of a very powerful sound in spite of the predominance of piano (p) markings throughout.

At the opening of the first movement, Beethoven included the following direction in Italian: "Si deve suonare tutto questo pezzo delicatissimamente e senza sordino" ("This whole piece ought to be played with the utmost delicacy and without damper[s]"[29]).

Ernst Oster writes: "With the aid of the Fantaisie-Impromptu we can at least recognize what particular features of the C♯ minor Sonata struck fire in Chopin.

Depeche Mode released a version of the piece, performed by Alan Wilder, as a B-side on their 1988 single Little 15.

[35] Psychedelic rock band Vanilla Fudge covered the piece (along with Fur Elise) that is part of "Phase Two" on their 1968 album, The Beat Goes On.