If subjecting war to law is one of the most important legal achievements of the 20th century, progressing further in that direction is one of the most important challenges for the 21st century. The problems it poses are many: the term “war” has formally fallen into disuse and we talk about “peacekeeping”; armies are today the product of cooperation between states and international organizations; private contractors increasingly participate in warlike activities, as the case of the Iraq war demonstrates; and the lines between war and very serious forms of crime (terrorism, organized crime) are increasingly blurred. This volume compiles the contributions presented at XVth International Congress on Social Defence, and tackle the criminal-legal issues raised by these new scenarios. It constitutes an innovative volume, gathering together the work of both academic and military authors, who have drawn on their theoretical and practical experience.