Under the M'Naghten rule, the accused must be clearly proved to have been laboring under a defect of reason at the time of the act so as not to know the nature and quality of the act or the wrongness of the act. In the 1950's, two competing formulations emerged: (1) the Durham test, under which criminal conduct is excused when it is the product of mental disease or defect and, (2) the American Law Institute's Model Penal Code, which introduced a volitional element. By the early 1980's, this latter formulation, which excused a defendant if he lacked a substantial ability to control his own behavior, had been adopted by a majority of jurisdictions. Reaction to the acquittal of John Hinckley for his attempted presidential assasination, changes in law regarding commitment procedures, and trends toward deinstitutionalization have led to reevaluations of the insanity defense by the APA, AMA, and ABA. All three agree that the present law ought to be changed and that it is a major mistake to allow either the jury or expert witnesses to speculate about whether a defendant had free will at a particular moment in the past. They also agree that (1) persons found not guilty solely because of mental illness should be subject to civil commitment to protect the public safety, and (2) the difficulties in predicting dangerousness do not require that acquitted defendants be treated the same as other mentally ill persons who have not committed serious criminal acts. Since the time of writing, the reforms discussed here have been embodied in the Insanity Defense Reform Act of 1984. Included are 16 references.