After four decades of steady growth, U.S. states' prison populations finally appear to be declining, driven by a range of sentencing and policy reforms. One of the most popular reform suggestions is to expand probation supervision in lieu of incarceration. However, the classic socio-legal literature suggests that expansions of probation instead widen the net of penal control and lead to higher incarceration rates. This article reconsiders probation in the era of mass incarceration, providing the first comprehensive evaluation of the role of probation in the build-up of the criminal justice system. The results suggest that probation was not the primary driver of mass incarceration in most states, nor is it likely to be a simple panacea to mass incarceration. Rather, probation serves both capacities, acting as an alternative and as a net-widener, to varying degrees across time and place. Moving beyond the question of diversion versus net widening, this article presents a new theoretical model of the probation-prison link that examines the mechanisms underlying this dynamic. Using regression models and case studies, I analyze how states can modify the relationship between probation and imprisonment by changing sentencing outcomes and the practices of probation supervision. When combined with other key efforts, reforms to probation can be part of the movement to reverse mass incarceration.