This chapter addresess the operation of reciprocity in the enforcement of international law. It looks at the roles reciprocity may play in the consequences of a breach of international law, analyzing the exception of nonperformance and State responsibility. In particular, it analyzes the consequences that attach to the breach of erga omnes obligations or peremptory norms, and the rules and limitations on countermeasures. The structure and mechanisms of international responsibility demonstrate the persisting importance of reciprocity in international law and the structural limitations that international law’s horizontal nature poses to establishing specific consequences for breaches of community obligations.