The insanity defense is a significant and challenging element of the criminal justice system, which permits defendants to claim that their serious mental illness made them unable to comprehend the nature or immorality of their conduct when they committed the felony. Here we discuss the progression of legal principles from ancient Roman law to the influential M’Naghten Rule, as well as the current standards, including, the Irresistible Impulse Test and Model Penal Code Test.