This chapter explores the development of international law by tracing its origins from the early days of the modern state system to the present. For many centuries Islamic law, in its several forms, served as international law in large parts of the world. The term international law, as well as the older term, law of nations, refers to a set of rules that are binding on international actors. For the emerging European system, the Peace of Westphalia meant the formal recognition of a new system of international relations. The concept of consent-based law is captured in the notion of legal positivism. With a growing acceptance of a positivist approach to international law, the international community recognized that there were two primary ways in which states could consent to create rules of international law: treaties and custom. The use of multilateral treaties to create rules of international law on a more general basis began in the nineteenth century.