A confluence of factors—a perfect storm—interfered with the intractable rise of imprisonment and contributed to the emergence of a new sensibility defining continued mass imprisonment as non-sustainable. In this context, reducing America’s prisons has materialized as a viable possibility. For progressives who have long called for restraint in the use of incarceration, the challenge is whether the promise of downsizing can be met. The failure of past reforms aimed at decarceration stands as a sobering reminder that good intentions do not easily translate into good results. Further, a number of other reasons exist for why meaningful downsizing might well fail (e.g., the enormous scale of imprisonment that must be confronted, limited mechanisms available to release inmates, lack of quality alternative programs). Still, reasons also exist for optimism, the most important of which is the waning legitimacy of the paradigm of mass incarceration, which has produced efforts to lower inmate populations and close institutions in various states. The issue of downsizing will also remain at the forefront of correctional discourse because of the court-ordered reduction in imprisonment in California. This experiment is ongoing, but is revealing the difficulty of downsizing; the initiative appears to be producing mixed results (e.g., reductions in the state’s prison population but increases in local jail populations). In the end, successful downsizing must be “liberal but not stupid.” Thus, reform efforts must be guided not only by progressive values but also by a clear reliance on scientific knowledge about corrections and on a willingness to address the pragmatic issues that can thwart good intentions. Ultimately, a “criminology of downsizing” must be developed to foster effective policy interventions.