Although customary international law (CIL) has been central to international law from its inception, it is often misunderstood. This edited volume remedies that problem by tracing the history of CIL and provides an in-depth study of its theory, practice, and interpretation. Its chapters tackle the big questions which surround this source of international law such as: what are the rules that regulate the functioning of CIL as a source of international law? Can CIL be interpreted? Where do lines between identification, interpretation, application, and modification of a rule of CIL lie? Using recent developments, this volume revisits old debates and resolves them by proffering new and innovative solutions. With detailed examples from international and national courts, it places CIL in a range of settings to explain, explore and reflect upon this developing and highly significant field. This title is also available as Open Access on Cambridge Core.