Presenting a thorough examination of intelligence activities in international law, Sophie Duroy provides theoretical and empirical justifications to support the cutting-edge claim that states’ compliance with international law in intelligence matters serves their national security interests. This book theorises the regulation of intelligence activities under international law, identifying three layers of regulation: a clear legal framework governing intelligence activities (legality); a capacity to enforce state responsibility (accountability); and the integration of legality and accountability into responsive regulation by the international legal order (compliance). The empirical relevance of these three layers of regulation is demonstrated through in-depth case studies of state responsibility in the CIA-led war on terror and an analysis of the accountability of Djibouti, the Gambia, Poland, the United Kingdom, and the United States for conduct in the CIA-led war on terror. Overall, the author shows that the most reliable path to long-term national security is the effective regulation of intelligence activities under international law. Making an original contribution to existing theories of compliance and regulation, as well as the law of state responsibility and its enforcement, this book will be essential for students and scholars of public international law, human rights, intelligence and security studies, and international relations. It will also be a valuable resource for practitioners of international law with an interest in intelligence, state responsibility, and terrorism and security law.