Customary International Law is the most contentious area in Public International Law, magnifying the ontological insecurities of International Law through its own apparently paradoxical structure. Customary International Law is an attempt to create normative structures to constrain states, but structures which are, themselves, drawn from the conduct of states. Custom has unique importance, as the only universally binding branch of international law. From the alleged prohibition on torture to the alleged legality of Humanitarian Intervention, claims of customary law are complex, increasing, and contested. The End of Customary International Law offers a fresh perspective on these endemic controversies. Operating at the theoretical level, this book locates the root causes of indeterminacy in the failure, or inapplicability, of orthodox legal theorising. Beginning with an evaluation of legal positivism, and a critique of the works of Hart, the book proceeds to examine the unsuitability of modern natural law approaches to the unique environs of international society. The extent to which all major theories of international law are undermined by the inadequacies of their municipal law antecedents is then explored. This analysis is enhanced by a study of Koskenniemi's critique of international law, a critique which is, in turn, situated in the competing desires for empirical accuracy or moral approbation. However, it is only when we abandon both empirical and moral theorising that the causes of indeterminacy in international law can be eliminated. The book concludes by outlining a concrete, theoretically coherent, and uniquely adapted theory of Customary International Law.