An examination of the elements of crime as established in modern penal codes and an assessment of the significance and effect of mental illness both suggest the inadequacy of proposals restricting defenses of insanity to the negation of the mens rea requirement as it is narrowly construed today. A study of the origins and development of the insanity defense shows that the Renaissance idea of individuality and the Enlightenment's emphasis on understanding as a precondition for responsibility combine to provide the essential preconditions for criminal punishment. The insanity defense developed as a device for precluding criminal responsibility where this condition of understanding was not fulfilled. The legal formulation of the defense of insanity has undergone numerous changes, but this study of its history, use, and justification concludes that the test should be based on a determination of the existence and effect of mental disease on the understanding and volitional aspect of personality. Approximately 600 references, a table of cases, and an index are provided.