International law, including human rights law, has expanded enormously in the past century. A growing body of anthropological research is investigating its principles and practices. Contemporary international law covers war and the treatment of combatants and noncombatants in wartime; international peace and security; the peaceful settlement of disputes; economic arrangements and trade agreements; the regulation of the global commons such as space, polar regions, and the oceans; environmental issues; the law of the sea; and human rights. This review demonstrates how anthropological theory helps social scientists, activists, and lawyers understand how international law is produced and how it works. It also shows the value of ethnographic studies of specific sites within the complex array of norms, principles, and institutions that constitute international law and legal regulation. These range from high-level commercial dispute settlement systems to grassroots human rights organizations around the world.