This chapter analyses existing practice concerning the backbone of the Rome Statute, part of an admissibility test to be assessed once the jurisdiction of the International Criminal Court (ICC) is established: the principle of complementarity. Its basic premise is that states are primarily responsible for the investigation and prosecution of crimes under international law, including war crimes. Thus, in ideal situation, should states carry their obligations perfectly, the principle would render the ICC superfluous. In a real world, however, the principle counts with states being “unwilling or unable” genuinely to carry out these obligations, and under those conditions, the ICC steps in. As such, the principle is a tool to solve vertical conflict of jurisdictions (domestic and international). As the ICC has exercised its jurisdiction for more than 22 years, it has already had an opportunity to elaborate upon the principle, yet certain unclarity remains. This chapter critically points out some problematic interpretations and deals with both theoretical, yet not addressed topics, as well as those already addressed by the ICC case law.