International law and armed conflict exist in a symbiotic relationship. In some cases, law shapes conflict proactively by imposing normative limits in advance of the appearance of proscribed conduct. Much more commonly, armed conflict either reveals lacunae in the law or demonstrates how law designed for yesterday’s wars falls short when applied to contemporary conflict. When that happens, international law reacts by allowing provisions to fall into desuetude, embracing new interpretations of existing prescriptions, or generating new norms through practice or codification. In the 21st Century, both international security and armed conflict are the subject of arguably unprecedented sea changes. As a result, claims that both the jus ad bellum and jus in bello are unwieldy and ill-fitting in the context of modern hostilities have surfaced prominently. Whether one agrees with such dire assessments, what has become clear is that armed conflict is increasingly exposing faultlines in the law governing the resort to force. The intent of this collection of essays in honour of Professor Yoram Dinstein on the occasion of his 70th birthday is to explore such faultlines, first by identifying them and then by assessing their consequences. In a sense, then, the essays, contributed by the top minds in the field, will serve to assist academics and practitioners to anticipate pressure on the law governing armed conflict and, to the extent possible, react accordingly. Paralleling Professor Dinstein’s classic works – War, Aggression, and Self-Defence and The Conduct of Hostilities Under the Law of International Armed Conflict − the book addresses both ius ad bellum and ius in bello topics.