Despite vast expenditures on prison construction in the late 20th century, infrastructure has not kept pace with the dramatic growth of incarceration in the U.S. As a result, extreme prison overcrowding has led to humanitarian, legal and fiscal emergencies nationwide. These emergencies are especially pronounced at the state level, where the Great Recession most directly affected and severely curtailed public spending; today, more than a third of state prison populations exceed institutional capacity. In the present policy environment, rather than investing scarce capital on building more prisons, state-level legal reforms aimed at downsizing the prison population are widely seen as the more prudent solutions. Little is known, however, about the diffusion and implementation of prison reform laws among local criminal justice actors and their effects at the county level of practice, where the incarceration process begins for most inmates. This project examines the 2011 “Realignment” of California’s unconstitutionally overcrowded prison system as an empirical window into how legal interventions and policy innovations filter to lower levels of government and diffuse into local organizational and professional practices. Using multiple methods, the study investigates how differences in local organizational culture shape the meaning of law on the ground in ways that bolster or undermine the reform goal of decarceration. The research focuses on two questions: (1) how do local criminal justice actors respond to, comply with, shape, and resist prison downsizing laws, and (2) what effect do these responses have on decarceration as a key metric of institutional change? A combination of group-based trajectory modeling and institutional ethnographic methods are used to assess the proposition that local organizational culture mediates the implementation of prison downsizing laws and that variation in county organizational culture explains differences in the outcome of decarceration. These multiple methods enable to the study to: (1) specify the measures of local variation most salient in predicting decarceration, (2) identify processes by which local organizational culture mediates law, as well as variations in these processes across counties, and (3) relate these variations to the outcome of decarceration.