The principle of complementarity provides a framework as to when the Prosecutor of the ICC may and should interfere vis-a-vis national judicial systems. The principle acknowledges the primary right of states to prosecute while also recognising the need for international interference when states fail in this task. As formulated in the Rome Statute, however, it leaves complex questions unresolved. To mention a few: When is a national criminal proceeding really an attempt to shield the perpetrator? When can a national judicial system be characterised as unavailable? And when will an ICC...