This article examines whether the Rome Statute is binding on individuals, i.e. whether its provisions which define international crimes are substantive or jurisdictional in nature. This question cannot be resolved by the text of the Statute alone. It is both vexing and fundamental, and has significant conceptual and practical consequences. If the Statute is only jurisdictional in nature, then the source of substantive norms of criminal law binding on individuals must be elsewhere, primarily in customary law. If this is so, then the Statute could never go beyond customary law, even though it arguably attempts to do so in several instances, and any individual accused before the Court should be able to mount a challenge as to whether the charges against him have a basis in customary law. If, on the other hand, the Statute is seen as being substantive in nature, then the Statute — a treaty — must be binding on individuals who have never consented to be bound by it, nor could have done so. Then it may well go beyond customary law, but it would potentially run afoul of the nullum crimen sine lege principle in two cases — when a particular situation has been referred to the Court by the UN Security Council or by a non-state party — since the supposedly substantive Statute could not have been binding on the individuals concerned at the time that they allegedly committed their offence. These issues have been already raised (in Lubanga, its first case), and will be raised before the Court, as with the declaration lodged by Palestine whereby it accepted the Court's jurisdiction for Gaza, or with regard to the situation in Darfur. The article argues that the best approach in such situations is for the Court to ‘read down’ the Statute so as to conform to customary law that was binding on the individuals in question at the relevant time. This, however, is not the only solution open to the Court, and the article examines others. In doing so, it also deals with broader issues of when and how treaties can directly bind individuals in international law.