The Insanity Defense of Not Guilty by Reason of Insanity is the defense used by some mentally disordered defendants who do not have the capacity for understanding right and wrong at the time of their criminal act. This defense has perplexed legal and mental health professionals for centuries. Though it has been a part of the legal system since the early Greeks, it has been and continues to be amended, modified, and in some jurisdictions, abolished. Moreover, despite its infrequent use, many in our society hold onto the belief that defendants can evade criminal responsibility by means of this defense. Furthermore, insanity is often equated with psychosis; however, the two are not the same and have different connotations. It is essential for any clinical psychiatrist to understand the concept of the insanity defense and how it differs from psychosis. This paper will clarify the meaning and purpose of the insanity defense and will differentiate insanity and psychosis.