AT the moment the International Criminal Court (ICC) lists the crime of aggression as one of the four crimes falling under its jurisdiction. Yet, until the elements of this crime can be agreed to, no prosecutions for this crime can be engaged in by the Prosecutor’s Office of the ICC.1 Preliminarily, it has been proposed that there are three elements to this crime: State aggression, individual actus reus, and individual mens rea. There is considerable controversy in international law today about how to understand the place of the act element in the crime of aggression, that is, in the crime for which individuals, as opposed to States, may be sanctioned for having initiated or waged aggressive war. I will discuss conceptual and normative problems in the act element that must be proven in order to hold individuals liable for the crime of aggression. Today this is an especially timely topic since a special committee drafting a definition of aggression and delimiting the elements of the crime of aggression for the International Criminal Court is supposed to issue its report by 2009. I try to provide a beginning understanding of the act element of this crime in what follows. In international criminal law it is individuals not States that are in the dock, and yet the crimes that are alleged are those that only States or other groups can commit. Concerning the crime of aggression, there are even larger problems than in other international crimes in order for trials to proceed. In the crime of aggression, the acts that individuals commit are not themselves criminal: they are only criminal because they are part of the initiation of illegal war. I attempt to solve this problem, but along the way I indicate how difficult the problem is and why it will often be very hard for individual prosecutions to go forward except of the highest-ranking State leaders. There is both a conceptual and normative question about how we link the act of an individual person to State aggression in a criminal trial. The conceptual question, the main one taken up in this article, relates to the fact that one individual cannot wage war. An individual’s act is always only one of many acts that constitute waging war. The conceptual question is why this individual’s act is singled out among so many other acts, and why this individual’s act is called the actus reus, the guilty act, of the crime of aggression. The normative question follows closely on the heels of the conceptual question. Why should we think that an individual’s act is sufficient to establish criminal liability for the crime of aggression? Even if we think that an individual’s act could be conceptually called an actus reus of some kind of crime, why think that this is normatively sufficient for establishing liability for something so enormous as the crime of waging aggressive war? Of course, in answering these questions it will be significant that we will almost always be looking at the acts of leaders rather than mere followers. The article proceeds as follows. In the first section, I briefly consider some lessons to be learned from the trials at Nuremberg, the only significant trials that we have had for the crime of aggression. Here I highlight one of the more interesting findings of these trials concerning the actus reus of individuals. In the second section, I explore an idea put forward by Roger Clark that the key to actus reus in this type of crime is to think of how the act relates to the circumstance or context of the overall war. Ultimately I agree with Clark that this is the right question to ask but disagree with his proposed answer to the conceptual question posed above. In the third section, I examine two of the main candidate acts, commanding and planning, that seem to be the best at linking individuals to the larger State aggression. In the fourth section, I argue that merely taking part in an aggressive war is not sufficient to establish the actus reus of the crime of aggression. In the final section, I discuss one important defense that is open to some State leaders, the defense of superior orders. Throughout this article, I try to find the most plausible basis for holding State leaders criminally liable when State aggression occurs. I argue that the actus reus element can be met by some individual military or political leaders but that it is far harder to do so than is normally thought.