Over the course of decades beginning in the 1970s, a combination of rising crime and escalating punishments resulted in unprecedented levels of incarceration throughout the nation. In Illinois, for example, per capita incarceration increased more than 500 percent between 1975 and 2015, and annual state corrections budgets grew roughly twenty-five-fold. By the end of that period, Illinois prisons were operating at 150 percent of design capacity, with nearly 50,000 individuals in custody. And “mass incarceration,” as the phenomenon had come to be known, was being regarded by leaders across the political spectrum as a harmful and divisive policy failure. As the bipartisan Illinois State Commission on Criminal Justice and Sentencing Reform concluded in 2016, “We incarcerate too many people and often punish people more than is necessary to serve legitimate public goals.” The Illinois prison population has fallen significantly since the Commission issued its report, but not necessarily due to any deliberate state reform. Long-term declines in arrests and criminal court filings, changes in sentencing practices across Illinois, and the impact of the COVID-19 pandemic on law enforcement and court operations have all been much more influential factors. Much of what really drives state imprisonment is decentralized local decision-making—a complex amalgam of thousands of arrest, charging, processing, and sanctioning choices—that resists simple analysis and easy solutions. That’s why finding the right balance of correctional responses to crime requires the active involvement of criminal justice researchers and evaluators. To reduce unnecessary incarceration, develop more effective and less harmful alternatives, and improve overall system performance and outcomes, policymakers and planners need a data-informed view of the way the justice system really operates and the outcomes it produces.