In this thesis, I investigate the adequacy of post-functional definitions of art, including institutional, historical, disjunctive and buck-passing definitions. I evaluate these definitions with respect to their extensional adequacy, intentional adequacy, and informativeness. Philosophers of art have not always made a clear distinction between definitions of art and theories of art. I argue that this distinction is important. A definition of art must provide the means to distinguish artworks from non-artworks. In contrast, a theory of art need not do this, but it should be informative about such matters as the production or reception of artworks. To clarify what a definition is, I explain the concept of essence and from there, investigate various accounts of essentialism and anti-essentialism about art. I conclude that none of the definitions of art so far proposed are adequate. However, I argue that we do not in fact need a definition of art. Theories of art are potentially of considerably more use than definitions.