Passacaglia and Fugue in C minor is an organ piece by Johann Sebastian Bach. Presumably composed early in Bach's career, it is one of his most important and well-known works, and an important influence on 19th and 20th century passacaglias: Robert Schumann described the variations of the passacaglia as "intertwined so ingeniously that one can never cease to be amazed."
The above text from the Wikipedia article "Passacaglia and Fugue in C minor, BWV 582" text is available under CC BY-SA 3.0.