It is notorious that international courts and tribunals have greatly contributed to the development of customary international law (CIL) by, for instance, articulating the constituent elements of custom and clarifying the conditions required for its modification. This volume demonstrates that they have also been actively engaged in the interpretation of CIL. In elucidating CIL interpretation before and by international courts and tribunals, the volume chooses three focal points: theory, method and normative interactions. Viewing CIL and its interpretation from these vantage points leads to a more complete picture of the role and function of CIL interpretation in international courts. The volume encourages readers to question orthodox theories on CIL and its interpretation, to look anew at what has long been labelled mere identification of custom, and to take a systemic approach to CIL, which, even in the process of interpretation, remains unwaveringly connected to treaties and general principles of law.

At first glance, one may think of international investment law as a response to custom (or lack thereof), instead of a field of its application. However, in fact, the opposite is the case. The interpretation and application of customary rules and principles are the bread and butter of international investment law and arbitration. With a diverse range of expert contributors, this collection traces how customary international law is practised in international investment law. It considers how custom should be interpreted and how its rules and principles should be understood and applied by investor-state arbitral tribunals. Raising and addressing vital questions surrounding custom and international law, this collection is a necessary contribution to the scholarship of the theory and history of customary international law and international investment law. This title is also available as Open Access on Cambridge Core.

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.