This new edition of Hugh Thirlway's authoritative text provides an introduction to one of the fundamental questions of the discipline: what is, and what is not, a source of international law. Traditionally, treaties between states, custom deriving from state practice, and general principles of law were seen as the primary means by which international law was created. However, whether this is still an adequate definition of the sources, and how they may operate in modern international society, has been questioned in significant ways. This book provides an insightful inquiry into all the recognized, or asserted, sources of international law. Some matters it investigates are the impact of ethical principles on the creation of international law; whether 'soft law' norms come into being through the same sources as binding international law; and the place of jus cogens norms, and those involving rights and obligations erga omnes in the creation of international legal norms. It studies the notion of 'general principles of international law', and the evolving relationship between treaty-based law and customary international law. Re-examining the traditional model, it investigates the increasing role of international jurisprudence, and the activities of international organizations and non-state actors as potential new sources.