Following the development of the International Criminal Court (ICC) the mental state of the perpetrators of genocide, crimes against humanity and war crimes will become a more important issue in regard to defence and mitigating factors. This article examines how the International Criminal Tribunal for the Former Yugoslavia (ICTY) in particular has dealt with the mental illness defence to date, and how its judgements can serve as guidance for the ICC as it becomes the major international court of the future. The absence of a mental health defence in the Statutes of the ICTY and the International Criminal Tribunal for Rwanda has led to a reliance on the Rules of Procedure and Evidence of the two tribunals. There are major difficulties in using the mental health defence as it is defined in the Statutes of the ICC because of a requirement for the destruction of mental capacity as a valid defence. Fitness to plead and the defence of intoxication are also examined.