Aleatoric music (also known as aleatory, indeterministic, or chance music) is music where some element of the composition is left to chance, or some primary element of a composed work's realization is left to the determination of its performer(s). The composer's creative input to the outcome of a piece is either greatly reduced or even removed completely. 'Creative input' includes melody, harmony, instrumentation, or even logical arrangement.

In aleatory music the players are encouraged to choose the sequence of movements, sections or individual chords and notes, or to improvise on a pattern or idea suggested by the composer--for example, in Ligeti's Aventures, on letters of the alphabet, and in Lutoslawski's Preludes and Fugue, on a series of lines, squares and triangles. Thus, the result differs from player to player and from performer to performer. Aleatory music is, in conception, similar to both Far Eastern art music and to jazz and rock, all of which involve improvisation, on an agreed basis, as a feature of the performance.

The term became known to European composers through lectures by acoustician Werner Meyer-Eppler at Darmstadt Summer School in the beginning of the 1950s. According to his definition, "aleatoric processes are such processes which have been fixed in their outline but the details of which are left to chance".

Among examples of aleatory music, Klavierstück XI (1956) by Karlheinz Stockhausen features 19 elements to be performed in changing sequences, certain orchestral works of Witold LutosÅ‚awski from after 1959 contain music where the orchestral ensemble is not precisely dictated, and in some works by Krzysztof Penderecki characteristic sequences are repeated quickly, producing a kind of oscillating sound.