Op. 36, commonly referred to as the Enigma Variations, is a set of a theme and its fourteen variations written for orchestra by Edward Elgar in 1898–1899. It is Elgar's best-known large-scale composition, for both the music itself and the enigmas behind it. Elgar dedicated the piece to "my friends pictured within", each variation being an affectionate portrayal of one of his circle of close acquaintances.

After its 1899 London premiere, the piece achieved popularity and was given international performances. The people portrayed in the variations include his wife Alice, Augustus J. Jaeger and Elgar himself. It has been arranged for various instruments. The enigma is not the identity of the persons portrayed, as those are known, but rather a hidden theme that is, in Elgar's words, "not played". This hidden theme has been the subject of much speculation, and various musicians have proposed theories for what melody it could be, although Elgar did not say that it was a melody. The enigma could be something else, such as a symbol or a literary theme. Elgar accepted none of the solutions that were put forward in his lifetime, and, pleased with his little joke, took the secret with him to the grave.