This timely book discusses the problem of State responsibility in connection with terrorist acts committed by non-State actors. It provides a detailed assessment of the consequences of wrongful acts of the State using contemporary examples such as the Bosnian Genocide, 9/11, and the 2016 and 2020 Nice attacks. State Responsibility and Terrorism underlines the conceptual foundations of State responsibility before expertly examining the consequences for the commission of terrorist acts, complicity, and the failure to prevent and punish these crimes. Also, the book explores how conditions for the lawful use of force change according to State involvement, simultaneously dissecting the feasibility of complicity between States and non-State actors for terrorist conduct. Incorporating relevant contemporary case law and literature, examples include the 1979 Tehran Hostages case and the 2005 London bombings. In this framework, chapters discuss the Articles on State Responsibility for Internationally Wrongful Acts and assess the erga omnes character of the prohibition of terrorism in international law. Innovative and insightful, this book is a crucial resource for students and academics in public international law, as well as terrorism and security law. It is also beneficial to practitioners in international dispute settlement for reference to recent debates on international terrorism and State responsibility.