The study concludes that what caused the prison boom of the last 30 years was persistently increasing crime rates, sentencing policies that put more offenders in prison and kept them there longer, and sufficient State revenues to pay for it all. Capital spending is an intervening variable in the analysis. Although it did not itself cause prison populations to increase, it made future increases possible. When direct and indirect effects are combined, eliminating current capacity as an explanation, the critical categories are crime and State spending, followed by sentencing policy, politics, and crowding. The principal drivers of prison population size are thus the apparent need for more prisons and the ability to pay for more prisons. Regarding implications of these findings, the author advises that the availability of publicly acceptable alternatives to incarceration may not be sufficient to reverse prison population growth; however, Federal funding of alternatives to incarceration, but not prisons, would provide States with the financial incentive to reduce prison populations while expanding alternatives to incarceration. The study relied on a panel of States over time, which significantly increased the sample size compared to previous studies and allowed for dynamic modeling over time. Consistent with previous studies, the prison population was measured as the jurisdictional population per 1,000 State residents, including not only prisoners in State facilities but also convicted offenders doing State time in local jails, private correctional facilities, Federal prisons, and facilities in other States. 6 tables, 6 figures, and 63 references