This book examines the development of the right of self-defence in international law both as an introduction to the subject and as a critical consideration of its central themes and debates. Special emphasis is laid on the development since 9/11. The right of self-defence is analysed from the point of view of international and constitutional law as well as from the perspective of state practice. The key question the book attempts to answer is whether a state can lawfully invoke ist right of self-defence to fight non-state actors. The book appeals to all international lawyers, academics, students, and practitioners, as well as those interested in politics and international relations.