Following a review of previous modeling efforts, another method for projecting prison populations is presented based on a familiar model of individual criminal careers. The implementation of the model is discussed using data from Pennsylvania. Projections are then made, using data from Massachusetts, Utah, and South Carolina to forecast prison populations through the year 2020. Four different amendments to current sentencing policy are also considered in terms of their effects on prison populations. Two of these changes involve increases in all prison terms, while two changes increase only the sentences given to older offenders. The projections based on the status quo in sentencing indicate a general upward trend in prison populations, even though the birth rate surge following World War II will not dominate future developments. Appendixes explain the importance of the average sentence length and present the status quo projections for Pennsylvania, Massachusetts, Utah, and South Carolina.