Necessity is a notoriously dangerous and slippery concept--dangerous because it contemplates virtually unrestrained killing in warfare and slippery when used in conflicting ways in different areas of international law. Jens David Ohlin and Larry May untangle these confusing strands and perform a descriptive mapping of the ways that necessity operates in legal and philosophical arguments in jus ad bellum, jus in bello, human rights, and criminal law. Although the term ""necessity"" is ever-present in discussions regarding the law and ethics of killing, its meaning changes subtly depending on the context.