It is common in international practice that several states and/or international organizations contribute together to the indivisible injury of a third party. Examples thereof are aplenty in relation to climate change and other environmental disasters, joint military activities and cooperative actions aimed at stemming migration. Such situations are hardly captured by the existing rules of the law of international responsibility. In particular, the work of the International Law Commission, which is widely considered to provide authoritative guidance for legal questions of international responsibility, has little to offer. As a result, it is often very difficult, according to the existing rules of the law of international responsibility, to share responsibility and apportion reparation between the states and/or international organizations that contribute together to the indivisible injury of a third party. The Guiding Principles on Shared Responsibility in International Law seek to provide guidance to judges, practitioners and researchers when confronted with legal questions of shared responsibility of states and international organizations for their contribution to an indivisible injury of third parties. The Guiding Principles identify the conditions of shared responsibility (including questions of multiple attribution of conduct), the consequences of shared responsibility (notably, the possibility of joint and several liability) and the modes of implementation of shared responsibility. The Guiding Principles are of an interpretive nature. They build on the existing rules of the law of international responsibility and sometimes offer novel interpretations thereof. They also expand on those existing rules, backed by authoritative practice and scholarship, to address complex questions of shared responsibility.