"Roughly 2.2 million people are locked up in prison or jail; 7 million are under correctional control, which includes parole and probation; and more than $80 billion is spent on corrections every year. Research has shown that policy changes over the past four decades have put more people in prison and kept them there longer, leading to exponential growth in the prison population even while crime has dropped to historic lows. But despite widespread agreement that mass incarceration is a serious problem, the national conversation is light on details about what it will take to achieve meaningful and sustainable reductions … To advance the policy conversation, decisionmakers and the public need to know the impact of potential policy changes. Our Prison Population Forecaster can estimate the effect, by state, of policies that aim to reduce prison admissions and length of stay for the most common types of offenses. The tool currently uses data from 15 states, representing nearly 40 percent of the national prison population, to forecast population trends and project the impact of changes on rates of admission or lengths of stay in prison … This forecasting tool paves the way for a more productive conversation about the need for tailored reforms that address the unique drivers of mass incarceration in each jurisdiction" (p.1). This website provides interactive access to these statistics comprising the Forecaster: select one of 15 states or all states; select offense/admission type—violent, nonviolent, property, drug, revocations, and all offenses; select policy change—reduce new admissions, and reduce length of stay; and state percent reduction—reduce by 5%, 10%, 25%, and 50%. The article looks at: the reforms needed to reduce mass incarceration at the state level; rethinking who goes to prison and how long they stay; and whether there is any low-hanging fruit left—more methods to reduce national prison populations.