International investment law is one of the most dynamic fields of international law, and yet it has been criticised for failing to strike a fair balance between private and public interests. In this valuable contribution to the current debate, Valentina Vadi examines the merits and pitfalls of arbitral tribunals' use of the concepts of proportionality and reasonableness to review the compatibility of a state's regulatory actions with its obligations under international investment law. Investment law scholars have hitherto given greater attention to the concept of proportionality than to reasonableness; this pivotal book combats this trajectory by examining both concepts in such a way that it does not advocate one over the other, but instead enables the reader to make informed choices. The author also explores the intensity of review as one of the main tools to calibrate the different interests underlying investor-state arbitrations. This timely book offers a useful conceptual framework for reconciling the opposing interests at stake, making it a valuable resource for international law scholars and practitioners and other interested readers.