Prelude (Toccata) and Fugue in (C or) E major, BWV 566 is an organ work written by Johann Sebastian Bach probably during his 4 month-stay at Lübeck or afterwards in the winter of 1705–1706. It comprises five sections and is an early^[1] work in grand form of Bach.

Its compositional form resembles that of Praeludia by Danish-German composer Dieterich Buxtehude.^[2] The first section alternates manual or pedal cadenzas with dense suspended chords. The second is a charming fugue with much repetition following the circle of fifths. The third section is a brief flourish for manuals, ending with an even briefer pedal cadenza punctuated with 9-voice chords. The fourth section, in ³
₄ time, is a second fugue with a rhythmic subject resembling the theme of the first fugue immediately followed by the fifth and final section that opens with a virtuous pedal-solo.

Bach also wrote a transposed version of the piece in C major (BWV 566a), to play on organs tuned in meantone where E major would sound discordant due to the tuning of the organ (with a very sharp D♯). Various recordings of the C major version exist mainly on historic instruments, for example Ton Koopman's recording on the Schnitger organ in Hamburg's Jacobikirche, and Marie-Claire Alain's recording on the Silbermann organ at Freiberg Cathedral. Both have a high pitch leaving the "concert" pitch up to a tone higher than modern pitch, where the temperament is significantly unequal to merit playing it away from E major. Modern organs or those tuned to a more equal temperament do not have this need.

The C major version is known nowadays through manuscript copies by J. Tobias Krebs and J. Peter Kellner.

• Other toccata and fugues

• Prelude (Toccata) and fugue in E major: Scores at the International Music Score Library Project
• Prelude (Toccata) and fugue in C major BWV 566a
• Free download of BWV 566 recorded by James Kibbie on the 1736 Erasmus Bielfeldt organ in St. Wilhadi, Stade, Germany