When is a norm peremptory? This is a question that has troubled legal scholars throughout the development of modern international law. In this work, Daniel Costelloe suggests - through an examination of State practice and international materials - that it is the legal consequences of a norm which distinguish it as peremptory. This book sheds light on the legal consequences that peremptory norms have, for instance, in the law of treaties, international responsibility and state immunity. Unlike their substance or identification, the consequences of peremptory norms have remained under-studied. This book is the first specifically on this topic and is essential reading for all scholars and practitioners of public international law.