The relatively young, albeit dynamic, field of international criminal law, spawned by the proliferation of international criminal courts and tribunals over the last two decades, has yet to consolidate its theoretical contours, in particular with respect to the clarification of major substantive law concepts. The doctrine of mens rea is among the fundamental concepts that ought to be clearly defined and construed for core international crimes such as genocide, war crimes, and crimes against humanity. With no general definition of mens rea in either the Charter of the International Military Tribunal (known as the Nuremberg Charter), the Statutes of the International Criminal Tribunal for the Former Yugoslavia (ICTY), or the International Criminal Tribunal for Rwanda (ICTR), only the Rome Statute of the International Criminal Court (ICC) introduced a comprehensive legal provision on the mental element of a crime. The judges of the Court breathed life in Article 30 of the Statute in its early jurisprudence. In broad terms, mens rea is a state of guilty mind that a person entertains while committing a crime. It is the one of the most fundamental doctrines in criminal law, be it national or international, since the absence of mens rea on the part of a perpetrator while committing a crime precludes the attribution of criminal responsibility. Although the law on mens rea in international criminal law takes inspiration from the laws and jurisprudence of common and continental law jurisdictions, the gravity, magnitude, and complexity of core international crimes impose an obligation to carefully consider how various mens rea standards are applied and construed in the international law context.