Before the International Criminal Court (ICC) came into being, world public attention was focused on issues such as the significance of the Court’s establishment, the importance of implementing international criminal justice and the time when the Rome Statute could enter into force. Once the Court was established, attention naturally turned to practical issues, such as whether it would be able to operate normally and perform its historic mission. The question of whether the ICC can operate effectively and perform its mission largely depends on the scope and degree of co-operation provided to it by states. This co-operation concerns not only states party to the ICC but also non-party states. This article offers to explore the obligation of non-party states to co-operate under international law, the prospects of their co-operation and the legal consequences of non-co-operation. The author suggests that beyond the general principle of the law of treaties according to which treaties are binding only on states parties, when viewed in the light of other general principles of international law, co-operation with the ICC is no longer voluntary in nature, but is instead obligatory in the sense of customary international law. Therefore, while a state may not have acceded to the ICC, it may still be subject to an obligation to co-operate with it in certain cases.