This thesis examines the attribution of criminal responsibility for the crime of aggression in international criminal law. Prosecuting aggression is predicated by the so-called leadership clause—an individual can be held responsible only if he or she meets the requirement of being in a position of control over or to direct state action. This ‘control or direct’ clause—which replaced the ‘shape or influence’ standard that was applied in Nuremberg—is based on a simple normative premise: individual responsibility ought to be restricted to state leaders and exclude followers. This work argues that the notion of leadership in this context denotes a normative determination that the individual had a decisive influence over the state policy on using armed force. To make such a normative determination, the inquiry needs to include the power dynamics of the given state so as to properly comprehend which structures of power govern state policies. This argument is built on the basis of the conceptual structure of the definition of the crime of aggression and an analysis of relevant case law and international documents. To this end, the thesis constructs a conceptual framework that provides for general conditions for criminal responsibility based on the material (actus reus) and mental (mens rea) elements of the crime of aggression. Against this background, the work critically appraises perpetration and complicity and concludes that individual conduct that, at the very least, significantly affects the state act of the use of armed force is the kind of behaviour prohibited by the crime of aggression.