The Statute required sixty ratifications or accessions for entry into force. The date of entry into force – 1 July 2002 – is an important one, if only because the Court cannot prosecute crimes committed prior to entry into force. Entry into force also began the real formalities of establishing the Court, such as the election of judges and Prosecutor. States were also invited to sign the Statute, which is a preliminary step indicating their intention to ratify. They were given until the end of 2000 to do so, and some 139 availed themselves of the opportunity. Even States that had voted against the Statute at the Rome Conference, such as the United States and Israel, ultimately decided to sign. Many of those which had abstained in the vote on 17 July 1998 also signed. States wishing to join the Court who did not deposit their signatures by the 31 December 2000 deadline are said to accede to, rather than ratify, the Statute.

Senegal was the first to ratify the Statute, on 2 February 1999, followed by Trinidad and Tobago two months later. The pace of ratification was speedier and more dramatic than anyone had realistically expected. By the second anniversary of the adoption of the Statute, fourteen ratifications had been deposited. By 31 December 2000, when the signature process ended, there were twenty-seven parties. On the third anniversary of adoption, the total stood at thirty-seven.