Fitness to plead is a fundamental legal concept. Its determination in England and Wales rests on professional interpretation of the 'Pritchard' criteria (1836). In the United States, the determination of the analogous concept of competence to stand trial rests on professional interpretation of the 'Dusky' criteria (1960). Numerous assessment instruments have been developed in North America to help guide professional determinations of competence to stand trial, but such assessments are not routinely employed in British settings. The evidence reviewed calls into question the utility of the fitness to plead construct as currently formulated and highlights the inadequacy of the procedures employed in its determination. We argue that both conceptual and procedural changes are required.