For decades, the field of international criminal justice was associated with the promise of new international criminal tribunals. Yet the creation of a permanent international tribunal in 2002 – the International Criminal Court – has turned attention to the fight against impunity at the national level. Complementarity, the jurisdictional framework at the heart of the Rome Statute, is usually credited with this development. Today, it is an article of faith that state- driven accountability, rather than international trials, is central to ending impunity. Activists speak of a Rome System of Justice, in which states are the first line of defense while the International Criminal Court, prodding states into action, intervenes only as a court of last resort. This thesis questions the widespread assumption that complementarity creates positive incentives for states to enforce international criminal law. Through an empirical analysis of when and why states prosecute international crimes, the thesis suggests that there are few causal connections between complementarity and governmental policies aimed at fighting impunity. Moreover, it argues that other jurisdictional frameworks are more likely to generate positive accountability incentives for international and state actors. The thesis concludes that, while the word complementarity aptly conveys the emergence of a new anti-impunity ideology premised on international-national cooperation, the actual jurisdictional framework of the International Criminal Court is unlikely to foster an effective system of international criminal justice.