This chapter examines the sources of international law. International legal rules are not as easily located as their domestic law counterparts. Whereas at the domestic level, only a relatively small number of bodies are endowed with law-making powers, at the international level, all states have law-making capacity. Moreover, state acts are not the only source of international legal rules. The result is a mosaic of law-making processes, forums, and regimes. The chapter focuses on the two most significant sources of international law: treaties and customary international law. It then turns to the relationship between international law-making and the principle of state sovereignty. Finally, the chapter considers the body of non-binding norms, which increasingly permeates and regulates all facets of international life. This so-called soft law takes many forms; it is often highly influential in its own right and may harden into binding law over time.