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 and state practice were seen as the primary means with which to create international law. However, more recent developments have recognized customary international law, alongside international treaties and instruments, as a key foundation upon which international law is built. This book provides an insightful inquiry into all the recognized, or asserted, sources of international law. It investigates 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 whether jus cogens norms, and those involving rights and obligations erga omnes have a unique place in the creation of international legal norms. It studies the notion of 'general principles of international law' within international law's sub-disciplines, 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 looks at the nature of international organisations and non-state actors as potential new sources of international law. This revised and updated book provides a perfect introduction to the law of sources, as well as innovative perspectives on new developments, making it essential reading for anyone studying or working in international law.