This book fills a major gap in the scholarly literature concerning international criminal law, comparative criminal law, and human rights law. The principle of legality (non-retroactivity of crimes and punishments and related doctrines) is fundamental to criminal law and human rights law. Yet this is the first book-length study of the status of legality in international law – in international criminal law, international human rights law, and international humanitarian law. This is also the first book to survey legality/non-retroactivity in all national constitutions, developing the patterns of implementation of legality in the various legal systems (e.g., Common Law, Civil Law, Islamic Law, Asian Law) around the world. This is a necessary book for any scholar, practitioner, and library in the area of international, criminal, comparative, human rights, or international humanitarian law.