The U.S. penal system, based on the rehabilitative model, is unjust, often characterized by unbridled discretionary sentencing and prosecution and excessive use of long prison terms. However, the main cause of the failure of the rehabilitative model is not its philosophy, but the lack of evaluation research directed at the operation and implementation of the model. This should be kept in mind by the present reformers of the penal system, who put forward the ideal of 'just deserts.' Despite the narrow grounds justifying a commensurate deserts penal policy, the ways punishments are administered may have consequences for specific and general deterrence, for rehabilitation efforts, and for other outcomes. For example, sentencing policies may influence the availability of rehabilitation programs or the extent to which they are used. Since legislatures are not always attentive to the empirical base for instituting reforms or attuned to the crucial importance of making evaluation research an integral part of the proposed reforms, the advocates of new penal policies must advocate the systematic evaluation of their proposals as well. Such circumspection is necessary since it is not known what the real consequences of a penal reform will be. Relevant literature is discussed, and 'references are included.