International criminal courts must not only decide the guilt or innocence of defendants in immensely serious cases, but also make good law in the process. To help them do so, these courts have turned to experts. This Article identifies a type of expert witness that, thus far, has escaped scholarly attention: the crime-definition expert. Crime-definition experts have provided expert reports and testimony to international criminal courts on the meaning of the very crimes with which defendants are charged, including genocide, forced marriage, and recruitment and use of child soldiers. This Article critically evaluates the risks associated with using crime-definition experts in international criminal trials. Ultimately, it concludes that crime-definition experts may help tribunals achieve the various aims of international criminal justice, but have the potential to impair defendants’ rights and impede the tribunals’ ability to advance expressive and restorative justice aims. It advocates judicious use, if any, of these experts and proposes measures to reap the most benefit from crime-definition experts while minimizing the risks inherent in their use.