The Complementarity Principle
How the ICC's relationship with national courts works and why complementarity is both the Court's greatest strength and limitation.
Last Resort, Not First Resort
Complementarity is the foundational principle governing the ICC's relationship with national courts. Under Articles 17 and 20 of the Rome Statute, the ICC can exercise jurisdiction only when national courts are 'unwilling or unable genuinely' to investigate and prosecute. This makes the ICC a court of last resort: it does not replace national justice systems but steps in only when they fail. The principle is written into the Statute's preamble, which recalls that it is the duty of every state to exercise criminal jurisdiction over those responsible for international crimes.
Unwillingness is assessed by examining whether national proceedings are designed to shield the person from responsibility, whether there has been an unjustified delay inconsistent with an intent to bring the person to justice, or whether proceedings lack independence or impartiality. Inability exists when a state's judicial system has substantially collapsed or is unavailable, for example due to armed conflict or institutional breakdown, so that the state cannot obtain the accused, the evidence, or otherwise carry out proceedings. Crucially, admissibility also requires sufficient gravity, filtering out cases that, while technically within jurisdiction, are not serious enough to warrant the Court's attention.
