Rules of customary international law provide basic legal protections to foreign investors doing business abroad. These rules remain of fundamental importance today despite the growing number of investment treaties containing substantive investment protection. In this book, Patrick Dumberry provides a comprehensive analysis of the phenomenon of custom in the field of international investment law. He analyses two fundamental questions: how customary rules are created in this field and how they can be identified. The book examines the types of manifestation of State practice which should be considered as relevant evidence for the formation of customary rules, and to what extent they are different from those existing under general international law. The book also analyses the concept of States' opinio juris in investment arbitration. Offering guidance to actors called upon to apply customary rules in concrete cases, this book will be of significant importance to those involved in investment arbitration.