This book traces the various uses of the concept of necessity in international law, with the goal of determining whether there is any over-arching unity to these uses across the sub-disciplines of international law. The authors not only discuss necessity in international humanitarian law (IHL) and jus in bello, but also aim to situate necessity as understood in IHL within a larger discourse of international law generally. They 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 necessity in international law has three different conceptions that cut across the various domains: necessity as exception, necessity as license, and necessity as regulation. In this book, the authors describe how these conceptions differ, and analyze them from a normative standpoint, arguing that necessity by exception requires principled restrictions (as found in international criminal law). They go on to explore necessity as a license in IHL, and offer an articulate and workable standard for its curtailment. Further, the authors' methodology is to interrogate the basic theoretical structure of the law through philosophical investigations, including the analysis of historical and contemporary "Just War Theory," and to determine whether the law ought to be revised or not.