The adoption of sentencing guidelines in a number of States has motivated a substantial body of research on the relationship between these policies and State-level sanctions. A 2002 study by Sorensen and Stamen addressed the impact of sentencing guidelines in sanctions in the States and concluded that these reforms were associated with a reduction in rates of prison admission and incarceration. The current study briefly reviews the findings of this study, and makes the argument for a longitudinal approach to the study of mandatory guidelines’ impacts. The author proposes that over time, mandatory guidelines that link sentencing decisions to correctional resources help to mitigate prison populations, while guidelines that do not tend to contribute to growth in that area. In addition, the author proposes that voluntary guidelines will not have a significant impact on either commitment or incarceration rates. To test these hypotheses, the data on prison populations from 1975 through 1998 were analyzed. The dependent variables were incarceration rate per 100,000 and the number of new admissions to prison per 100,000. Results of the analyses suggest that mandatory guidelines have increased both commitment and incarceration rates in States where sentencing decisions are not resource driven. On the other hand, when mandatory guidelines were linked to capacity and expenditures, these policies had either a negative or an insignificant impact on prison populations. These findings suggest that cross-sectional analyses are an inappropriate and possibly misleading method for assessing the impact of sentencing policies. They also suggest that a closer examination needs to be made of the language and theories of punishment used to justify State-level guidelines. For policymakers, these findings suggest that they must consider the disparate impacts of different types of guidelines schemes when considering sentencing reform. 2 tables, 11 notes, and 39 references