This systematic analysis of State complicity in international law focuses on the rules of State responsibility. Combining a theoretical perspective on complicity based on the concept of the international rule of law with a thorough analysis of international practice, Helmut Philipp Aust establishes what forms of support for wrongful conduct entail responsibility of complicit States and sheds light on the consequences of complicity in terms of reparation and implementation. Furthermore, he highlights how international law provides for varying degrees of responsibility in cases of complicity, depending on whether peremptory norms have been violated or special subject areas such as the law of collective security are involved. The book shows that the concept of State complicity is firmly grounded in international law, and that the international rule of law may serve as a conceptual paradigm for today's international legal order.