The United States' high incarceration rate gets a lot of attention from scholars, policy makers, and the public. Yet the most common form of criminal justice supervision is not imprisonment but probation--and that is just as true for juveniles as for adults. Probation was originally promoted as an alternative to imprisonment that would spare promising individuals from the ravages of institutionalization. Instead, it often serves as a net-widener, expanding formal supervision to low-level cases. Like mass incarceration, mass probation is marked by deep racial and class disparities, and it can have devastating consequences for poor and minority communities. In this review, the author covers three aspects of probation supervision: (1) who is sentenced to probation; (2) what they experience; and (3) when and why probation is revoked (that is, when probationers are sent to jail or prison for violating the terms of supervision). The author then presents policy recommendations for each of these three stages that could reduce the harms of mass probation. They include scaling back the use of probation, offering probationers more meaningful help to improve their lives, and raising the bar for revoking probation. Though probation reform may not be a cure-all, it could reduce the scale of the criminal justice system and temper its detrimental effects.