This book offers a unique comparison between state and individual responsibility for international crimes and examines the theories that can explain the relationship between these two regimes. The study provides a comprehensive and systematic analysis of the relevant international practice from the standpoint of both international criminal law, and in particular the case law of international criminal tribunals, and state responsibility. The author shows the various connections and issues arising from the parallel establishment of state and individual responsibility for the commission of the same international crimes. These connections indicate a growing need to better co-ordinate these regimes of international responsibility. The author maintains that a general conception, according to which state and individual responsibility are two separate sets of secondary rules attached to the breach of the same primary norms, can help to solve the various issues relating to this dual responsibility. This conception of the complementarity between state and individual responsibility justifies co-ordination and consistent application of these two different regimes, each of which aims to foster compliance with the most important obligations owed to the international community as a whole.