The mentally disordered offender is a psychotic or retarded person incapable of making responsible decisions and is identifiable legally and psychiatrically. The mentally abnormal offender is the legally sane person who has a personality or behavioral anomaly warranting special attention. Independent surveys show that the mentally disordered constitute about 5 percent of the Canadian Federal offender population, and the mentally abnormal group, 25 percent. Public perceptions of mentally disordered offenders as unpredictable and dangerous and typically perpetrators of bizarre and violent crimes constrain expenditures for treatment services. The usefulness of psychiatric examinations in the prediction of dangerousness is questionable. Such assessments are more useful in determining the treatment potential of a case. The deinstitutionalization of patients in the mental health system has shown significant impact on the criminal justice system, as such patients, through their deviant behavior, have contacted the criminal justice system in their communities. Research on various target groups, such as the retardate, psychopath, and sexual offender, is encouraging and is producing apparently effective treatment modalities. Tabular data and about 40 references are provided.