Since the 9/11 attacks, international organizations have become actively engaged in devising counterterrorism strategies and frameworks. This monograph examines the role UN organs can play in implementing the law of State responsibility in global security contexts, using transnational terrorism as its principal case study. The institutional mechanisms utilized by the UN in implementing State responsibility are assessed in detail, shedding light on how the ICJ, the General Assembly and the Security Council contribute to the implementation of State responsibility in the context of global security. By acknowledging the Security Council's role as a post-9/11 legislator, this book argues that the Council can play an important and sometimes determinant role in implementing a State's legal responsibility for failing to prevent terrorism, both inside and outside the Chapter VII framework. Featuring a discussion of the more controversial consequences flowing from State responsibility, this monograph also explores the prospect of injured States adopting forcible measures against responsible States for their failures to prevent terrorism. The book investigates whether self-defence and other forcible reactions, envisaged both inside and outside the Council, can be reconciled with State responsibility principles.