Insanity Plea – A plea of not guilty based on a lack of criminal capacity. Not guilty by reason of insanity is a criminal plea essentially admitting the defendant committed the act of the offense yet denying responsibility because the defendant lacked the capacity to act with criminal intent at the time. A defendant may enter such a plea, which may be accepted by the court, or the defendant may have an instruction on such a plea submitted to a jury, which may render it as its verdict.

In the federal courts and most state courts, a judgment of not guilty by reason of insanity requires a commitment to a mental institution for an unspecified time for treatment. A finding of not guilty by reason of insanity operates as a final verdict, and jeopardy attaches to it for the purposes of later criminal charges from the same conduct.