Historically, insanity had no fixed medical definition. It was more a factor in determining punishment than in deciding guilt. The modern insanity defense in the Anglo-American world largely derives from a bizarre English murder trial in 1843, in which a deluded Daniel M'Naghten assassinated the secretary to the prime minister. His defense informed the jury that the human mind is not compartmentalized, and that a defect in one aspect could easily affect other areas. Following the man's sentence to an asylum, English judges formulated the M'Naghten Rule, which (according to the author) reduced the issue from one of legal responsibility to a medical one. There has been subsequent experimentation with other concepts, notably the irresistible impulse and the Durham test. The latter focuses on mental illness rather than mental state. A variant of the American Law Institute's (ALI) insanity proposal, now used in nearly all Federal circuits, extends the defense beyond cognitive defect to include volition as well. Currently, there is some confusion regarding insanity and criminal intent, also known as mens rea. These are not inseparable issues, but within the call for abolition of the insanity defense are call for abolishing mens rea. Mens rea is a critical legal concept that plays an important role in the dispensing of justice. Abolition of mens rea would reduce all crimes to ambiguous bodily acts. The rationale for the insanity defense is that the law should punish only controllable misdeeds. Five recent efforts to redefine the test for insanity are insanity as an affirmative defense, the bifurcated trial, guilty but mentally ill, abolition of the defense, and keeping the defense and abolishing the test. Footnotes and an index are included.