Scholarship on the expansion of the U.S. carceral state has primarily focused on imprisonment rates. Yet the majority of adults under formal criminal justice control are on probation, an "alternative" form of supervision. This article develops the concept of mass probation and builds a typology of state control regimes that theorizes both the scale and type of punishment states employ. Drawing on Bureau of Justice Statistics data from 1980 and 2010, I analyze whether mass probation developed in the same places, affecting the same demographic groups and driven by the same criminal justice trends, as mass imprisonment. The results show that mass probation was a unique state development, expanding in unusual places like Minnesota and Washington. The conclusions argue for a reimagining of the causes and consequences of the carceral state to incorporate the expansion of probation.