Reformers across the political spectrum are calling for a rollback of mass incarceration. The U.S. rate of incarceration in state prisons would have to decline by 75% to return to its 1970s level. How might this be accomplished? This Article provides descriptive statistics about the mix of offenses, sentence lengths, and admission types and shows that no single approach can undo mass incarceration. Those classified as violent offenders are a majority of those in prison, but nonviolent offenders are a majority of those entering, leaving, or having been in prison. A majority of those in prison are scheduled to be released within five years, meaning that steep reductions in prison admissions can have a large impact on imprisonment rates. Revisiting the sentences and parole options for those who have already been in prison ten years or more could have some impact. An examination of the rate of returns to prison after a first release from prison suggests that the rate of committing a new crime is low and that reductions in revocations for violations of the conditions of supervision are an important avenue for reducing incarceration. The U.S. states vary greatly in their mixes of prisoners by offense, sentence length, and returns to prison for parole violations with no new crime as well as in their histories of trends over time. States will vary markedly in which reforms will affect their prison populations, and assumptions based on old data may not hold true as conditions change.