After a long period of relative stagnation, substantive international criminal law has been invigorated primarily by the activities of the International Criminal Tribunals for the former Yugoslavia and Rwanda. Both ad hoc tribunals have made immense advancements to this area of international criminal law by, for instance, laying down detailed rules on what constitutes culpable conduct and when responsibility should be attributed for the conduct of others. These important advances notwithstanding, much remains in flux. The elements of the core international crimes are still subject to controversy. Theories of individual criminal responsibility, such as command responsibility and joint criminal enterprise, are highly controversial. There is as yet no knowledge of how international offenses should be graded according to different levels and degrees of culpability and harm. This book brings together a team of researchers and practitioners from the field of international criminal law, concerned with a new international agenda of refining substantive international criminal law. The diverse topics examined include the superior orders defense, the mental element, the defense of mistake, command responsibility, the crime of aggression, and the principle of legality.