This report of the National Council on Crime and Delinquency maintains that the preactive approach to prison population planning should be abandoned because it is often based on inaccurate predictions of the projected prison population. The proactive approach should be used instead, based on influencing policy factors that affect the number and rate of prison admissions (e.g., arrest and prosecutorial practices, diversion), and length of incarceration (e.g., speed of hearing or trial, sentencing practices). These factors depend on broader policy issues (e.g., public policy on crime and incarceration, financial resource availability and allocation), some of which are controllable. Proactive planning should proceed from the identification of controllable factors, through an assessment of realistic control potential, to the choice of those factors most likely to produce the desired end. Possible ways to assess potential are described.