Research has begun to systematically assess the relationship between sentencing policies and State incarceration rates. Prior studies, however, have examined policy-based relationships in isolation, failing to consider the impact of combinations of policies. Using a pooled time series design, this article examines interactions between structured sentencing, determinate sentencing, and State incarceration rates between 1978 and 2004. Results show that constraining release discretion through determinate sentencing matters more than constraining sentencing discretion through structured sentencing. Consistent with prior research, determinate sentencing was associated with lower incarceration rates independent of other policies. Contrary to prior research, however, the presence of presumptive sentencing guidelines was associated with lower incarceration rates only when combined with determinate sentencing. These findings suggest that while a State may effectively insulate sentencing decisions from outside social forces, if it fails to insulate release decisions from those same forces, they will continue to affect imprisonment levels. Tables and references (Published Abstract)