The question of what is, and what is not, part of international law is of course fundamental. Traditionally, treaties between states and custom (state practice) have been seen as the primary means by which international law is created. These two sources, along with the 'general principles of law', are specified in the Statute of the International Court of Justice (Article 38), and this text has long been treated as generally authoritative. However, whether this is still an adequate definition of the sources of international law, and how they may operate in modern international society, has been questioned in significant ways. Taking Article 38 ICJ Statute as starting-point, this book provides a careful assessment of all the recognised, or asserted, sources of international law. Among the issues considered are: the impact of ethical principles on the creation of international law; the existence of peremptory norms (those of jus cogens), and whether they come into being through the same sources as other norms; the place of these, and of norms involving rights and obligations erga omnes, in the operation of international legal relationships; the definition and role of 'general principles of law'; whether any of international law's sub-disciplines involve the application of additional sources; and the continuously evolving relationship between treaty-based law and customary international law. Re-examining the traditional model, the work takes account of the increasing role of international jurisprudence, and looks at international organisations and non-state actors as potential new sources of international law. The 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 any field of international law.