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.

‘The current volume is a thought-provoking publication that challenges preconceived ideas in relation to custom and international investment law. It analyses the continued relevance of custom and its interpretation in investment law. This publication not only broadens knowledge of investment law but also constitutes a very significant contribution to further understanding of intricacies and mysteries of customary international law. Authors of chapters are leading scholars and practitioners in field of international and investment law. They have dealt with complex and challenging aspects of uneasy relationship between international investment law and customary international law in an outstanding way. This volume is very highly recommended for everyone who is interested in challenging legal problems and evolution of international law.’

Malgosia Fitzmaurice - Queen Mary University of London

‘What, might one think, could international investment law, one of the ‘hottest’ topics in international law and practice today, have to do with international customary law - traditional and venerable certainly, but also old-fashioned and always number two among international law's sources? The answer given by this timely book is: a lot, with custom mostly ‘embedding’ investment law comfortably, but in some instances also clarifying it or even challenging it, to put it mildly. Just think: are we to accept that tribunals composed of three private lawyers may confirm, interpret or even change customary standards of\" behaviour by way of developing what they would like to see as FET or as rules of state responsibility?’

Judge Bruna Simma - Iran-United States Claims Tribunal