This chapter examines how complementarity has evolved from a legal rule of admissibility - an organizing principle for the regulation of concurrent jurisdiction - to an instrument of policy. This policy, often referred to as ‘positive complementarity’, is one that promotes the International Criminal Court (ICC) and the ‘Rome Statute System’ as proactive agents for domestic accountability. Drawing on constructivist international relations literature, the chapter traces this more ambitious articulation of the ICC’s relationship to national jurisdictions and argues that its ascendance reflects the work of norm entrepreneurs and transnational civil society networks who, through a duty-based reading of the Statute, have progressively sought to articulate a more catalytic vision for the court and, with it, a broad array of policy goals. At the same time, this duty-based interpretation has contributed to an increasingly disciplinary approach to implementation, one that privileges a mirroring of the Rome Statute’s content and the ICC’s form. Complementarity’s evolution is thus testament to the significant influence of non-state actors at both the transnational and local level, and of a growing effort on their part to route the entrenchment of domestic human rights through the framework and standards of international criminal law.