This thesis is structured around three key phases in the process of collaborative music-making - composition, preparation, and performance - examining the function of indeterminacy at each stage, and the way in which musical factors are determined. At what point in the creative process a musical decision is made, the path chosen, and critically, by whom the decision is taken, are all explored in the context of a portfolio of pieces performed and recorded as part of this practice-led research. The portfolio comprises recordings of projects undertaken with composers, as well as pre-existing repertoire, and the written commentary explores my creative role as a performer in relation to that of the composers and the other performers I have worked with. Practical issues faced in collaboration, practice, and performance are dealt with, as are questions of musicality, and the notion of success in musical performance.