Aleatora is a programming environment for making music that changes. Aleatora represents musical compositions as streams of computation, meaning that a piece of music can change each time it is played - the music may take different paths with different outcomes. Aleatora aims to make it easy for composers to weave dynamic materials into compositions, so they may be influenced by the outside world; e.g. the state of a random number generator, live input from a performer or an audience, or real-time data fetched over a network. The composer may build up a computational composition from smaller pieces, as streams may be combined by sequential, parallel, or functional composition. Compositions may also be self-referential, referring back to any saved point in their own history to use as a stream in the future, with potential divergence. Finally, Aleatora enables the composer to work at multiple levels of abstraction, from the level of audio samples up to notes, events, patterns, and beyond, all within the same compositional framework.