Decision-making about fitness to stand trial and the consequences of a finding of unfitness are fundamental to the integrity of any criminal justice system. They create thresholds for when mentally and physically unwell people are mandated to participate in criminal proceedings and they address the outcomes of such decisions for unwell accused persons. The jurisprudence relating to fitness to stand trial under international criminal law has particular challenges and complexities. The origins of contemporary controversies and the bases for modern decisions lie in rulings by the Nuremberg and Tokyo tribunals in the immediate aftermath of the Second World War. The decisions relating to Gustav Krupp, Rudolf Hess, Julius Streicher and Shumei Okawa wrestled with issues that have since recurred in respect of how trial systems should respond to unwellness going to the heart of whether persons can participate meaningfully in their own trials but dealing too with the temptation for persons accused of matters as serious as crimes against humanity and genocide to malinger, exaggerate symptomatology and to generate delays for strategic objectives.