This is a difficult term to define, since it pertains to such a wide variety of diverse musical practices, but most simply put is the act of musical decision-making during performance. From the perspective of Western musical traditions, it will frequently relate to relationships between decisions made by composers (and normally written down through musical notation) and those made by performers as part of their interpretation of a composition. Decision-making may relate to micro-level aspects of the musical surface, or macro-level aspects of structure and form. Improvisation may rely upon some degree of pre-determined framework (whether notationally or otherwise culturally mediated). During the twentieth century significant musical developments for which improvisation was integral were jazz, and subsequently free jazz (which loosened use of frameworks of harmonic progression, melodic development, relationships between performers, and musical temporality) along with some forms of (experimental) rock. The experimental music movement that emerged in the UK during the 1960s, privileged improvisation with social and political, as well as sonic, motivations. Improvisation, too, was an influence upon the development of American minimalist music. The term free improvisation is used to describe performance that makes extremely limited use of pre-made decisions or frameworks, and is (in principle at least) completely stylistically open-ended.

As in other forms of contemporary music, improvisation in electroacoustic music may be closely related to forms of aleatory or indeterminacy. So-called live electronics is now long established and diverse. Improvisation and aleatory often meet where musicians have built custom electronic instruments and devices, for example in the ‘composing inside electronics' aesthetics (associated with David Tudor), and performance with ‘feedback instruments'. The digital connectivity offered through computer technologies has allowed musicians to experiment with new modes of collective and collaborative decision-making on stage. Also, improvisation can be found in activities outside of conventional performances, for example in the use made by many composers of analogue studio equipment in shaping sound materials, and the decision-making made by participants in real-time activity with sound across distributed networks. Improvisation is also an integral aspect of DJ culture and turntablism.