In our post-11 September world, challenges to international peace and security emanate from non-State actors as never before. Under international law States have an obligation to act with due diligence in confronting non-State actors that engage in terrorism. The author of this book examines the grounds and mechanisms through which a State can bear responsibility for breaching its due diligence obligations in this regard. He explores the question whether a comprehensive definition of terrorism exists and reviews the development of the due diligence principle during the last century. After doing so, the author examines how the due diligence principle operates in the counter-terrorism context by analysing international and regional treaties and Security Council Resolutions. Theoretical issues that arise when interpreting the due diligence principle are also studied. The author concludes by critically engaging with the question whether national security should trump human rights in the fight against terrorism. This book fills a significant gap in the literature. It is principally designed for policy makers, academics, and students of international law.