The alarming rise in international terrorist activity since the attacks of September 11th has generated intense debate about the principles by which State responsibility for private acts of terrorism is determined in international law. This book analyses the legal principles that govern State responsibility for the conduct of non-State actors and examines their relevance and effectiveness in an increasingly interconnected world. The book argues that traditional approaches to determining the State's legal responsibility for violations of these counter-terrorism obligations lack conceptual coherence, fail to enhance State accountability for protecting civilians from private violence and are poorly suited to coping with today's terrorist threat. Drawing on a wide array of precedents and legal sources, the study offers a novel approach to regulating State responsibility for private violence that is grounded in principles of causation. The book argues that causation-based State responsibility for terrorism can be reconciled with the International Law Commission's Draft Articles on State Responsibility and offers a conceptual and legal framework that is better equipped to deal with the subtle interactions between State and non-State actors that make contemporary terrorism possible.