There are several tests for insanity throughout various U.S. jurisdictions: (1) the M'Naghten rules, the irresistible impulse test, the New Hampshire or Durham test (the product test), and the test recommended by the American Law Institute's Model Penal Code. Under the M'Naghten rule, it must be proved that at the time of the offense the accused had a mental defect of the mind that prevented an awareness of the criminal nature and quality of the act, or if the nature of the act was known, it was not perceived as wrong. The irresistible impulse test applies to an accused who may know the nature and quality of the offense and be aware that it is wrong but who may be irresistibly driven to commit the act by an overpowering impulse resulting from a defective mental condition. The product rule states that a verdict should be not guilty by reason of insanity if the offense resulted from or was the product of a mental disease in the defendant. The Model Penal Code has met one of the main objections to the M'Naghten test: its failure to recognize certain gradations in mental impairment. It accepts the theory that combines M'Naghten and the irresistible impulse test. Due to the large amount of criticism of these insanity tests, a new approach has been developed called the partial insanity or diminished capacity defense. In cases where the defendant acted from insane delusions or partial insanity arising from a diseased mind, a defense is afforded where the imagined facts would have afforded a justification for the act if real. Support for the diminished responsibility doctrine is already widespread and is continuing to grow. Nine references are provided.