Chapter 25 relates to what is probably the most recognized, but perhaps the most rare, evaluation performed by a forensic professional. The pursuit of an insanity defense today is challenging and, in some states, not even permissible. These cases illustrate the evolution of the various state standards for establishing insanity (and therefore acquittal) for criminal defendants with mental illness. The following cases are included in this chapter: M’Naghten’s Case, Durham v. U.S., Washington v. U.S., Frendak v. U.S., Jones v. U.S., U.S. v. Torniero, Foucha v. Louisiana and Clark v. Arizona.