The insanity defense is a legal construct that excuses certain mentally ill defendants from legal responsibility for criminal behavior. This practice guideline has delineated the forensic psychiatric evaluation of defendants raising the insanity defense. The document describes acceptable forensic psychiatric practices. Where possible, standards of practice and ethical guidelines have been specified. And where appropriate, the practice guideline has emphasized the importance of analyzing the individual case, the jurisdictional case law and the state (or federal) statute. This practice guideline is limited by the evolving case law, statutory language and legal literature. The authors have emphasized the statutory language of current legal standards, as well as the state or federal courts' interpretation of those standards because the same statutory language has been interpreted differently in different jurisdictions. Similarly, this practice guideline has reviewed the state and federal trends that determine which diagnoses meet the criteria for mental disease or defect. These trends yield to jurisdictional court interpretations. Finally, the authors hope this practice guideline has begun the dialogue about formulating a forensic psychiatric opinion by surveying the various approaches used to analyze case data. The forensic psychiatrist's opinion in each case requires an understanding of the current jurisdictional legal standard and its application, as well as a thorough analysis of the individual case. The psychiatrist's analysis and opinion should be clearly stated in the forensic psychiatric report. It should be noted that the role of a psychiatric expert witness in the criminal justice system is predicated on the law's interest in individualizing the criteria of mitigation and exculpation. Forensic psychiatric analyses and formulations of opinions are, therefore, subject to change as the legal guidance changes.