A few years before the expected convening of the First Review Conference on the Statute of the International Criminal Court, the Special Working Group for the Crime of Aggression has made very significant progress in preparing the ground for enabling the Court to exercise its jurisdiction over what the Nuremberg Tribunal famously called the ‘supreme international crime’. The complex structure of this absolute leadership crime has been fully explored, including its implications for the interplay between the definition of the crime and the general principles of international criminal law. At the same time, the crucial analytical distinctions between state and individual conduct as well as between the substantive elements of the crime and the possible procedural role of the UN Security Council appear to be generally accepted. In the light of the momentum achieved on the diplomatic level, the First Review Conference presents a distinct and historic window of opportunity to the world's political leadership to complete the Rome Statute and thereby prevent its prominent lacuna from becoming permanent and thus turning into a legitimacy gap. The final phase of the negotiations should be guided by two principles. First, the substantive definition of the state act of aggression should stay within the legitimate limits of international criminal justice by not exceeding undisputed general customary international law. Second, a possible procedural role for the Security Council must not have the practical effect of placing the permanent members of the Security Council beyond the reach of the law.