A Canadian judicial decision recently held that a person convicted of a criminal offence who suffered a substantial deterioration in mental condition since the trial could be found unfit to be sentenced. The court based its conclusion on both historical arguments and the Canadian Charter of Rights and Freedoms. There are compelling justifications for recognizing this concept. The paper looks at the history of fitness and how the sentencing phase became disconnected from claims of unfitness in the late 19th century. It then considers theoretical justifications based on fairness, viewing sentencing as a moral discourse, and the effect of the Canadian Charter of Rights and Freedoms. Because of the number of practical questions that need to be addressed before implementing a concept of unfitness at the sentencing stage, the paper looks at some common law jurisdictions for guidance: Australia, New Zealand, and the American states of New York, Illinois, Connecticut, and Ohio. From these comparisons comes the idea of a "provisional cap". That is, the recognition of unfitness at the sentencing stage should be followed by a form of sentencing that takes into account the gravity of the offence, the prosecutor's position, any relevant aggravating or mitigating factors that can be adduced, and then results in a "provisional" sentence, whether custodial or community-based, which stays in effect until the offender becomes fit. The paper ends with a model that incorporates this approach while providing both that offenders will be confined, if necessary, in hospitals and not prisons, and also that the dispositions will be reviewed annually to ensure that the least restrictive and least onerous sanctions are imposed.