The recognition of individual criminal responsibility under international law is relatively recent. The commission of mass atrocities during the 20th century prompted the international community to recognize that individuals can be criminally responsible directly under international law and to work for the establishment of an international criminal court having jurisdiction on international crimes committed by individuals. Thus, after World War II, the principle was established that individuals—and not only states—can be the addressee of obligations, commit crimes, and therefore bear criminal responsibility directly under international law. As affirmed by the judges sitting in Nuremberg: “Crimes against international law are committed by men, not by abstract entities, and only by punishing individuals who commit such crimes can the provisions of international law be enforced.” As a consequence, it is now undisputed that individuals shall be punished for the commission of crimes under international law (or “international crimes”) that seriously damage the interest of the international community as a whole, so that the goals of prevention and deterrence can be achieved. This principle is now well expressed in the Preamble of the Rome Statute of 1998, where it affirms that “the most serious crimes of concern to the international community as a whole must not go unpunished” and that the International Criminal Court aims “to put an end to impunity for the perpetrators of these crimes and thus to contribute to the prevention of such crimes.” The attribution of criminal responsibility to individuals does not exclude that states can be held responsible for the violations of international law that also potentially amount to international crimes; however, individual criminal responsibility under international law possesses the same legal nature as the criminal responsibility under domestic law, whereas the responsibility of states is of an international/civil nature. Given the macro-criminal dimension of the crimes at stake, which normally involve the state apparatus and are committed by an organized group or in a systematic manner, the process of “individualization” of the responsibility encounters more than one challenge. First, the issue of immunities for heads of states and other subjects under international law; second, the regulation of the modes of liability, which need to take into account the collective dimension of commission of international crimes. To overcome some of the difficulties, the rules of attribution of criminal liability to individuals had been partly reinterpreted and new modes of liability developed. Moreover, the principle of personal culpability excludes collective and strict liability. As a consequence, several grounds to exclude criminal responsibility are recognized. Finally, the enforcement of individual criminal responsibility for international crimes is the real challenge in a context of collective commission and macro-dimension of the crimes, where, moreover, the mechanisms of enforcement are not homogeneous.