This essay reviews trends since the early 1980s in the number of inmates confined in American prisons as well as possible factors contributing to the massive increase in prison admissions (ranging from highly functionalist structural accounts to more culturally embedded midrange ones). Defining features of the late twentieth century imprisonment boom are discussed, encompassing global notoriety; persistent racial disparities; the role of felony drug filings, convictions and sentences in fueling both the scale and racial disparities of imprisonment; and regional and jurisdictional variations in trends across three planes: federal-state, interstate, and intrastate. Finally, the recent “stabilization” of incarceration rates in the United States is described and possible implications considered.