This book provides the first comprehensive analysis on the shared international responsibility of multiple states and international organizations that contribute to a single refoulement. It develops a set of Guiding Principles on the determination of shared responsibility for multi-actor contribution to a refoulement, based on the progressive development of the law of international responsibility. The book initially explores how destination states engage with transit states and international organizations (the European Union, NATO, the United Nations and the International Organization for Migration) in refoulements. It then analyses why existing legal instruments, notably the International Law Commission’s Articles on International Responsibility and the Guiding Principles on Shared Responsibility in International Law, fail to address such complex scenarios. Building on this critique, the book proposes new Guiding Principles and demonstrates how they can be applied in practical scenarios, offering guidance for scholars, practitioners, judges and policymakers working in human rights and refugee law.