The Statute of the ICTY incorporates the customary international law principle of superior responsibility. Where a superior knew or had reason to know that subordinates were about to commit or committed a crime, he/she is under an obligation to take preventative or punitive measures against subordinates. The mens rea, or knowledge, of a superior is pivotal in determining his/her criminal responsibility. However, despite the customary law status of the principle of Superior Responsibility, the definition and standard of mens rea applied has varied. The ICTY Trial Chambers in the Čelebići and Blaškić cases reached opposite findings on the definition of the term “had reason to know” under customary law. The Appeals Chamber has rendered its definition of mens rea. However, the standard applied appears inconsistent with the principle of Superior Responsibility.