It should be made clear that the International Criminal Court does not replace the national jurisdictions of the States Parties. The Statute asserts that this Court carries out its functions in accordance with the principle of “complementarity”. This means that the ICC is complementary to the judicial systems of the States Parties. And the Statute emphasizes that “it is the duty of every State to exercise its criminal jurisdiction over those responsible for international crimes”.
Furthermore, Article 17 of the Statute specifies the situations where the Court can hear cases. These situations are limited to if the State Party to the ICC is determined to be unwilling or unable to prosecute the alleged criminal activity domestically. If this proves to be right, then the Court could exercise jurisdiction over the criminal act in question.
To give more elaboration, a case will be inadmissable, and the Court will not be able to exercise its jurisdiction, if a national authority is investigating or prosecuting the case or has already done so, unless the circumstances indicate that the State is nevertheless unwilling or unable to carry out proceedings genuinely.
Consequently, the crimes that fall within the jurisdiction of the ICC must and will still be prosecuted at the national level, that is to say before the national courts, unless these courts are found or determined - by the ICC - to be unwilling or unable to carry out their duty.