Founded in 1967, the Amsterdam-based improvising collective the Instant Composers Pool is one of the longest consistently performing groups in improvised music. This thesis forms an ethnography and musicological study of the ICP Orchestra, which originated when the "pool" developed a more coherent line-up around 1980. With a background in experimental music as well as free jazz, their performance practice differs in many respects from the practices of American forms of jazz. Whereas most accounts of improvisation emphasise orality and creative interaction in opposition to the performance of composed music, 'instant composition' defines improvisation precisely in terms of compositional thinking. Moreover, founding member and orchestra leader Misha Mengelberg composed a very diverse repertoire for the group which draws on styles from Duke Ellington to John Cage and uses various forms of compositional and notational techniques to explore the different improvisatory possibilities that they afford, thus blurring the distinction between improvisation and composition both in name and in practice. Apart from a detailed historical and ethnographic description of a group that is central to a genre that has been underrepresented in music-historical research, this thesis investigates the repertoire of the ICP and its use as an opportunity to reconsider the relation between musical text and performance. Drawing on my observations and interviews with the musicians, and connecting these to theories of material culture and science and technology studies, it develops a concept of compositions as animated and animating objects in performance, tools and materials that participate in the creative interactive process of improvised performance rather than textual representations of 'the music itself'. I substantiate this theory with detailed descriptions of ICP performances recorded during fieldwork. This contributes to a rethinking of musical notation and simultaneously brings new insights into improvisation as a creative practice.