The federal prison population exceeds 218,000, a tenfold increase since 1980. This massive growth is projected to continue and is accompanied by increasing costs, which account for 25% of the Department of Justice's budget and edge out other important public safety priorities. This brief describes the main drivers of the federal prison population, half of whom are drug offenders. Front-end decisions about who goes to prison and for how long have the greatest impact, suggesting that reductions in sentence lengths -particularly for drug offenders - can most directly contain future growth. "Back-end" changes, such as increasing earned credits for early release, can also help alleviate the pressure. The federal system can learn much from state efforts to contain prison populations and costs; doing so will require the cooperation and support of numerous players across all branches of the federal system.