The International Court of Justice has observed that international law is not a static set of rules, that it undergoes ‘continuous evolution’. The evolution of international law is a subject that has absorbed international lawyers for centuries, for, among other things, the way in which law develops and changes clearly determines the rules that are applicable today. This book addresses one particular characteristic of the evolution of international law, namely that it does not occur in a legal vacuum, but is instead circumscribed and regulated by fundamental rules, principles and processes of international law. One such process is the process of customary international law, which is also referred to here as the ‘customary process’. This process governs how one particular kind of rules – rules of customary international law – is developed, maintained and changed.

Unlike treaty rules, which result from formal negotiation and explicit acceptance, rules of customary international law arise out of frequently ambiguous combinations of behavioural regularity and expressed or inferred acknowledgments of legality. Despite (or perhaps because of)their informal origins, rules of customary international law provide substantive content to many areas of international law, as well as the procedural framework within which most rules of international law, including treaty rules, develop, exist and change. Customary rules are particularly important in areas of international law, such as State immunity and State responsibility, where multilateral treaties of a general scope have yet to be negotiated.