This thesis explores the evolution of the insanity defense throughout legal history beginning with ancient Greek and Roman times. Ideas about treating the insane separate from the sane in a criminal proceeding were first expressed by famous philosophers such as Plato and Aristotle. The insanity defense was codified into the Justinian Code under Roman Law, but there was no criteria to distinguish who was insane and who was not. From the 14th to 19th centuries, a number of insanity tests were developed in English common law, resulting in the milestone M’Naghten rules, which became the basis for the insanity defense as it exists in the United States today. This paper explores how M’Naghten can be interpreted, what it does well, and its criticism. The thesis then explores how a number of other insanity defense standards rose in the United States, including the Irresistible Impulse Test, the New Hampshire test, the Durham test, the Model Penal Code, the Insanity Defense Reform Act, Guilty but Mentally Ill, and abolishing the insanity defense all together. The thesis asserts why all of these standards fall short of providing adequate protections for the insane in the criminal justice system and do not accurately define legal insanity. There is an analysis of both the theoretical and practical implications of trending alternate proposals for the insanity defense, including the Mental Illness Contribution Defense and Not Criminally Responsible By Reason of Recognized Medical Condition. Then, an argument is presented for the proposal for a new standard for insanity incorporating the ideas of philosopher Herbert Fingarette.