This thesis is the first full-length dissertation to focus on the music of John Zorn (b. 1953). One of the most prolific composers, performers, and impresarios of the New York downtown scene, Zorn's output traverses a wide range of musical practices: from free improvisation to music for jazz and rock groups, and pieces for classical musicians. The project locates his music in its historical and aesthetic context, and attempts to establish a critical framework for the analysis and close study of this diverse output, tracing the connections between the bewilderingly various areas of this music making. The first chapter introduces the performative conception of composition and identity that underscores the case studies of individual works in the subsequent chapters. Chapter two focuses on Spillane, a work of aural cinema for improvising musicians; the third chapter investigates issues of postmodern textuality in Carny, a fully notated work for solo piano; while the final chapter negotiates the undecidable terrain of Zorn's recent violin concerto, Contes de Fées. These pieces demonstrate how Zorn's output mobilises a continuum of practice that encompasses improvisation and notation, performance and text, freedom and control, and dissolves many of the pre-made categories of musicological discourse; I aim in this way to address the broader critical questions Zorn's music raises for musicology, particularly in relation to common assumptions about modernity and postmodernity. Zorn's often contradictory comments about his music form a constant counterpoint to the project, constructing a public persona that is as complex and conflicted as his musical output: at the same time as acknowledging the surface diversity and drama of his music, he insists on the importance of unity and integrity to his work. This is a music that stages a confrontation between worldliness and immanence, between heteronomy and autonomy, opening up the critical territory explored throughout this project.