There is little evidence that psychiatrists or other mental health experts can predict the future dangerous conduct of patients or prisoners with any substantial degree of certainty. Several legal issues arise with respect to Part XXI proceedings of the Criminal Code of Canada. The major legal obstacle to the imposition of an indeterminate sentence is the reliance in a Part XXI proceeding upon the predictive 'evidence' of psychiatrists and other experts. Canadian courts tend to assess predictions of future behavior on their individual merits and to let the problem of the unreliability of psychiatric predictions be construed as an issue of weight rather than admissibility of evidence. Dangerous offenders should be reliably and consistently identified and incarcerated to protect the public, but the length of imprisonment should be approximately the same for similar offenses. Initial reform of Part XXI requires clarification of language describing dangerous offenders and reduction of the special labeling of sexually dangerous offenders. A combination of proposed options for dealing with the difficulty of predicting dangerousness would be a determinate extended sentence added to that justified by the last offense, after which the Crown could seek further periodic detention. It would be understood, however, that the offender had already served his due sentence for the crime and is entitled in principle to be free. Chapter footnotes are provided.