The essay argues that the absence of an international treaty definition of aggression in international law should not preclude the prosecution of its perpetrators. Two legal regimes of responsibility, namely the prohibition against aggression as an international wrongful act and the crime of aggression have been entangled. Once one separates the criminal liability of individuals from state responsibility, a definition of the crime of aggression can be seen. According to the author, the contours of such a new definition contain the requisite degree of certainty for judicial approaches instead of merely political approaches. Consideration is also given as to whether conspiracy to wage a war of aggression may also be regarded as a separate crime within international criminal law.