The International Criminal Court (ICC) was established in 2002 to combat impunity for the most serious crimes of international concern. It seeks to do so in two ways: through a series of high-profile cases in The Hague, intended to deter future war criminals; and through its complementarity mechanism, which equips national legal systems to prosecute ICC crimes domestically. Through a case study of the prosecution of sexual and gender-based violence in the Democratic Republic of Congo, this article examines efforts by various stakeholders to realize the legal complementarity principle embedded in the Rome Statute. The article argues that the domestic prosecution of ICC crimes requires developments in four distinct areas: legislative reform, institutional reform, education and training, and the building of public trust and participation. The research also reveals that where developments in these areas have occurred, they have been propelled by a variety of domestic and international stakeholders. However, the ICC itself has failed to contribute significantly to the realization of complementarity that is central to achieving its mandate.