This article addresses why mental disorder is relevant to criminal responsibility. It begins by considering the meaning of criminal responsibility because it is impossible to understand the relevance of mental disorder unless one understands responsibility clearly. The next section provides a theoretical account of responsibility and excuse in general and addresses common misconceptions about these topics. The third section examines in detail why mental disorder can sometimes produce either a complete or partial excusing condition, such as legal insanity or partial responsibility,'' and whether the U.S. Constitution requires the provision of an excuse based on mental disorder. The section proposes that mental disorder should produce an excusing condition in appropriate cases. The next section considers the relation of mental disorder to mens rea, the mental state element'' that is a deยฎnitional criterion of most crimes, and whether the U.S. Constitution requires that defendants be permitted to use evidence of mental disorder to negate mental state elements of the crime charged. This section argues that mental disorder rarely negates mens rea, but in those cases in which a plausible claim for negation could be made, defendants should be allowed to make this claim.