This book aims to trace the various uses of the concept of necessity in international law, with the goal of determining whether there is any overarching unity to these uses across the subdisciplines of international law. The authors not only discuss necessity in international humanitarian law and jus in bello, but also aim to situate necessity as understood in IHL within a larger discourse of international law generally, and to untangle the confusing and often inconsistent usages of the term “necessity” in these broad areas of international law, including human rights law. The authors argue that the concept of necessity in international law has three different conceptions that cut across the various domains of international law: necessity as exception, necessity as license, and necessity as regulation. First, the book explores how these conceptions differ in a descriptive way. Second, the authors analyze these conceptions from a normative standpoint, arguing that necessity by exception requires principled restrictions (as found in international criminal law). Third, necessity as a license in international humanitarian law should be curtailed, though perhaps not as radically as some philosophers believe. The book offers an articulate and workable standard for this curtailment. Further, the authors’ methodology is to interrogate the basic theoretical structure of the law, and then use philosophical investigations, including the analysis of historical and contemporary Just War theory, to determine whether the law ought to be revised or not.