The size of the yearend Federal prison population grew by 77% over the 1998-2010 period, from 104,413 offenders who were convicted of federal crimes to an all-time high of 184,809. Drug offenders made up the largest portion of the increase in Federal prisoners, followed by weapon, immigration, and non-regulatory public-order offenders. An increase in the length of time to be served by prisoners was the leading contributor to growth, accounting for 58% of the total prison population growth between 1998 and 2010. Longer expected lengths of stay for drug offenders, alone, accounted for one-third of total growth in the prison population. Changes in federal conviction, investigation, and sentencing practices, respectively, also added to the prison populationnotably, a higher conviction rate in drug cases and heightened enforcement of immigration and weapon offenses. By contrast, prison population growth during this period was moderated by changes in the rate at which sentenced offenders were admitted to prison and modest declines in the federal prosecution rate. Report findings were based on a statistical decomposition analysis using data from the Bureau of Justice Statistics' Federal Justice Statistics Program.