Although the insanity defense is probably the most controversial of all criminal defense strategies, it is also, somewhat ironically, one of the least used. On many occasions when it has been used, particularly in the much-publicized 1984 acquittal of John W. Hinckley, Jr. for the attempted assassination of President Ronald Reagan, the insanity defense has tended to provoke public debate.

The insanity defense asserts that a criminal defendant should not be found guilty due to the defendant's insanity. The theory behind the defense is that a person who is insane lacks the intent required to perform a criminal act because the person either does not know that the act is wrong or cannot control his or her actions even when the person understands that the act is wrong. This theory is controversial because insanity itself is difficult to define, and the circumstances in which insanity can be used to excuse criminal responsibility are difficult to characterize.

The insanity defense has existed since the twelfth century, but initially it was not considered an argument for the defendant to be found not guilty. Instead, it was a way for a defendant to receive a pardon or a way to mitigate a sentence. The idea that insanity could bar the conviction of a defendant arose in the early nineteenth century in The Medical Jurisprudence of Insanity by an influential scholar named Isaac Ray, as well as in the seminal decision in England called the M'Naghten case.