The Safety and Justice Challenge is providing support to local leaders from across the country who are determined to tackle one of the greatest drivers of over-incarceration in America—the misuse and overuse of jails. These leaders—representing different cities, counties, and states with unique issues and situations—understand that this crisis took years to create and that only forward-thinking, smart solutions developed and implemented from the ground up will end it. With a five-year, $148 million investment by the John D. and Catherine T. MacArthur Foundation, jurisdictions selected through a competitive process receive financial and technical support in their efforts to rethink justice systems and implement data-driven strategies to safely reduce jail populations. The John D. and Catherine T. MacArthur Foundation’s Safety and Justice Challenge recognizes that there are better, fairer, and more effective alternatives to excessive jail incarceration. Key to beginning and sustaining reform efforts is an understanding of how jail use has changed, and what impacts this growth has had on individuals, communities, and the economy. For local leaders involved in the Challenge, this means a commitment to identifying the drivers of over-incarceration within their cities, counties, and states; engaging a diverse set of community stakeholders to determine ways to address local drivers of over-incarceration, address racial disparities in incarceration, and improve the system as a whole; and building infrastructure to track the right data and measure performance over time. Jurisdictions participating in the Challenge will develop and model effective ways to keep people out of jail who don’t belong there, more effectively reintegrate those who must be confined into the community upon release, and help them stay out of jail thereafter. In doing so, they will demonstrate alternatives to incarceration as usual, creating models for reducing unnecessary jail use to make communities healthier, fairer, and safer.