The monograph reviews Canada's dangerous offender legislation, which came into effect in 1977 and requires that two psychiatrists provide expert testimony and allows participation in dangerous offender hearings by psychologists and criminologists. These provisions not only cover dangerous behavior generally, but extend to sex offenders. A series of recent studies on the prediction of dangerous behavior are surveyed, as are contemporary treatment approaches for sex offenders. This chapter concludes that there is little evidence to suggest that psychiatrists or other mental health experts can predict the future dangerous conduct of patients or prisoners with any substantial degree of certainty. Moreover, ad hoc interviews with some 40 Canadian forensic psychologists, psychiatrists, and criminologists showed that these professionals themselves claimed little ability to predict the future violent behavior of dangerous offenders of the kind handled by Part XXI. The literature also suggests that more progress has been made in the assessment of sexual anomalies than the treatment of them. The monograph analyzes legal issues which arise in Part XXI proceedings, notably the admissibility of expert testimony as to future dangerousness and implications of the Canadian Charter of Rights and Freedoms. A concluding discussion of policy issues recommends clarifying the language in Part XXI that describes dangerous offenders and reducing the labeling of sexually dangerous offenders. Other suggested reforms address the inherent unreliability of psychiatric expert testimony on dangerousness and the management of offenders identified as dangerous. References accompany each chapter.