This book provides an authoritative account of international law. It preserves and extends Antonio Cassese’s exceptional combination of a historically informed, conceptually strong, and practice-infused analysis of international law, comparing the treatment of most issues in classical international law with the main subsequent developments of this constantly evolving field. Part I of the book covers the origins and foundations of the international community. Part II is about the subjects of the international community, including States, international organizations, individuals, and other international legal subjects. Part III examines the main processes of international law-making and the normative interactions between different norms, of both domestic and international law. Part IV studies the mechanisms of implementation of international law, including State responsibility, diplomatic and judicial means of dispute settlement, and enforcement mechanisms. Part V covers a number of areas which have undergone particular development and reached a high level of specialization, namely, UN law, the law governing the use of force, international humanitarian law, international human rights law, international criminal law, international environmental law, and international economic law (trade and investment).