A Theme and Variations is a theme, followed by any number of variations, which are different arrangements of the initial theme. Most commonly, the melody line is varied, by adding extra notes around the theme, perhaps in a semi-quaver pattern when the original was in crotchets or quavers.

In the Early Music and Renaissance periods, the theme may stay constant for one voice part throughout, while the other parts perform the variations. This was called cantus firmus. In the Baroque period, the bass line may remain constant while the top parts weave around in harmony with it. This was called a ground bass. Also in the Baroque, the harmonic sequence may remain constant, while the top line varies, but always in harmony.  (There is some similarity between this and Traditional or Dixieland Jazz).

In the classical and romantic periods, variations were usually on the melodic line itself. These may be faster decorations weaving around the tune, or a paraphrase of the tune, or a change between major and minor keys, and so on. This category is described further under variations form.