This chapter examines debates surrounding the nature and efficacy of modern international law. It begins by discussing the reasons why international societies construct institutions, and why different sorts of institutions have emerged in different historical contexts. It then considers the nature and origins of the modern institution of international law, its relationship with the practice of multilateralism, and the recent cosmopolitanization of the global legal order. It also explores the laws of war and concludes with an overview of different theoretical approaches to international law such as realism, neoliberal institutionalism, and constructivism. Two case studies are presented: the first is about whether international law is an expression of Western dominance and the second is about individual criminal accountability in non-Western countries. There is also an Opposing Opinions box that asks whether international law has any real effect on the nature and conduct of international relations.