The article briefly describes the evolution of the plea 'not guilty by reason of insanity (NGI)' in the modern Anglo-American system and focuses on the nature of the defense today -- how frequently it is raised and what effect a successful NGI plea is likely to have on a client. Then, the article offers an approach to the preparation and presentation of such a defense, which is unlike any other because the trial does not just involve the evaluation of the questions of whether the act was done and mens rea existed, but also a social assessment of whether the defendant should be held accountable. Thirty-nine notes are listed. (Author abstract modified)