A critical review of the insanity defense focuses on the M'Naghten rule, the American Law Institute test for insanity, selected Idaho cases, and United States v. Brawner. It concludes that the insanity defense did not produce accurate and reliable results or significant benefits for either society or criminal defendants. Idaho's abolition of the insanity defense shifts the principal emphasis on mental condition from the trial stage to the disposition stage of the criminal proceeding. In sentencing, mentally disordered offenders are to be committed to an appropriate official for placement in an appropriate facility, and the time in treatment is to count against a definite sentence. The act provides a nonexclusive list of criteria to guide the court at sentencing and makes several procedural changes. Under the new law, evidence of mental impairment is no more complicated than any other evidentiary question relating to the ability of the accused to form the necessary intent to commit the crime. The jury will simply be instructed on the elements of the crime, including the element of intent, and responsibility for dealing with the effects of the offender's mental illness falls on the sentencing judge. A review of U.S. Supreme Court decisions suggests that there is no constitutional impediment to abolition of the insanity defense. The article provides 90 footnotes.