In this article, we argue against testifying to the ultimate legal issue. There is no basis in science or clinical knowledge for determining the degree of capacity that is required in order to reach the threshold of capacity associated with legal questions such as competency and criminal responsibility. Society qualifies mental health professionals to provide expert testimony on the basis of their clinical and scientific expertise. Thus testifying to the ultimate legal issue is inappropriate and illogical and to do so threatens the integrity of mental health professions and the legal process.