This article first addresses the persistent confusion between causation and excuse. It demonstrates that causation is not the equivalent of compulsion and that causation per se is not an excusing condition. Then the article examines the conceptual and practical difficulties presented by the excuse that is variously labeled "compulsion," "involuntariness," "volitional problems," "irresistible impulse," and the like. It concludes that this excuse, when produced by internal causes, is far less well understood and assessed than forensic clinicians usually assume and that most such excusing conditions are better understood and assessed in terms of rationality problems.