As ICTJ co-hosts a discussion on complementarity on the margins of the Assembly of State Parties (ASP) of the International Criminal Court (ICC), the principle of ensuring accountability for serious crimes has seen a major breakthrough at a recent high-level meeting at Greentree. The meeting brought together international justice actors, development practitioners, UN representatives, and national rule of law actors to discuss the practical implementation of complementarity and how to strengthen domestic systems seeking to investigate serious crimes.