The basic moral issue in the insanity defense is whether it is just to hold responsible and punish a person who was extremely crazy at the time of offense. The defense is rooted in moral principles of excuse that are accepted in both ordinary human interaction and in criminal law, i.e., that there is no just punishment without desert, and no desert without responsibility. Arguments against the moral basis of the defense tend to confuse causation with excuse or moral and legal concepts with medical concepts. Other arguments against the defense, such as that it produces wrong verdicts or that assessment of past mental state is too difficult, also fail to convince. Such objections either are based on false empirical assumptions or incorrect logic or they fail to provide objections specific to the insanity defense. Alternatives such as mens rea, sentencing discretion, and the guilty-but-mentally-ill verdict are misguided. It is argued that the insanity defense is morally necessary and that substantive and procedural reforms can yield a limited and just insanity defense. In such reform, the criteria for defense, the role of mental health experts, the burden of persuasion, and postacquittal problems are issues requiring attention. A total of 168 footnotes are provided.