One etiological theory attributes prison population patterns to demographic factors (e.g., age, racial, and sex distributions as well as migration patterns) and economic factors, especially the unemployment rate. Prison population projections based on this etiology use mathematical equations to link demographic and economic changes with prison population changes. The advantages of this projection method are the reliability and accessibility of demographic data, the relative simplicity of the analysis, and the acceptance of this method among corrections administrators and legislators. The weaknesses of this method are the unreliability of economic data and its failure to consider factors within the criminal justice system. Another etiological theory of prison population patterns focuses on criminal justice system policy and practice as major factors influencing prison populations. Three features characterize the method of prison population prediction derived from this etiology: (1) projections depend on policy directions, (2) multiple projections are developed for various eventualities, and (3) projections are for the short term. The advantage of this method is its attention to the impact of policy decisions; its disadvantage is the difficulty of accurately assessing policy impacts and the cost of data collection and analysis. A draft working paper for the Prison Overcrowding Project to help guide policy groups in planning for policy implementation is included.