The study estimated the impact of sentencing guidelines on prison populations by using the multiple time-series design, which many consider the best procedure for evaluating State laws when a random experiment is not feasible. The multiple time- series design combines data from all States over nearly two decades. Dependent variables in the regressions are prison admissions and population in each States, and the independent variables include dummy variables that represent the sentencing guidelines laws. Findings show that sentencing guidelines are associated with declines in prison population growth in the six States where legislators decreed that guideline framers consider prison capacity when establishing guidelines for prison sentence lengths. In some States, the guideline laws alone apparently have caused prison population growth to moderate, but in others the guidelines were probably only one aspect of a larger policy to limit prison population growth. 2 tables and 58 footnotes