The aim of this thesis is to increase our ontological understanding of musical works in two ways. We'll increase our understanding of their categorial nature and we'll increase our understanding of what the identity of each musical work consists in. In chapter 1, I introduce the basic question of the thesis: what are musical works? This question is broken down into four separate questions which guide the structure and argument of the thesis. One question asks if musical works exist, the other asks about the ontological category to which musical works belong. In the first half of the thesis, I argue that musical works exist and that the best explanation of their features is given by assigning them to the category of universals. I argue for this conclusion by elaborating and defending a view according to which musical works are properties. I then show that this conception is superior to the strongest rivals. One rival takes musical works to be historical individuals (Rohrbaugh 2003). The other takes them to be actions of composition (Davies 2004). In chapter 6, I turn to two questions about identity. One question asks about the identity criteria for musical works, the other question asks for an explanation of musical work identity. In the remainder of chapter 6, I argue by reference to contextualist intuitions that a superficially appealing view, sonicism is problematic. The critique of sonicism is used to draw out certain contextualist assumptions that our ontology of music ought to explain. In chapter 7, I examine a family of Levinsonian contextualist proposals. I argue that the original formulations (Levinson 1980) give incorrect results, and that attempts to modify them (Davies, S: 2001) slip into obscurity, or undermine the aims of the project (Levinson 1992). I end chapter 7 by presenting the performance theory in a more flattering light. I argue that as a theory of musical work identity, the performance theory is the best on the market because it overcomes the problems associated with the Levinsonian views and deepens our understanding of contextualist intuitions. In chapter 8, I present an account of the ontology of music which integrates the property theory of category with the performance theory of identity by construing musical works as impure relational properties. I then defend the integrated account against various objections. In this defence, I elaborate a view of compositional actions as belonging to the ontological category of processes.