The prison time actually served by a convicted criminal depends to a significant degree on decisions made by the state during the course of imprisonment—notably, on whether to grant parole.We study a model of the adjustment of sentences assuming that the state’s objective is the optimal deterrence of crime. In the model, the state can lower or raise a criminal’s initial sentence on the basis of deterrence-relevant information obtained during imprisonment. Our focus on sentence adjustment as a means of promoting deterrence stands in contrast to the usual emphasis in sentence adjustment policy on avoiding recidivism.