--- - |2+ Research Summary Our study represents the first effort to evaluate systematically Proposition 47's (Prop 47's) impact on California's crime rates. With a state‐level panel containing violent and property offenses from 1970 through 2015, we employ a synthetic control group design to approximate California's crime rates had Prop 47 not been enacted. Our findings suggest that Prop 47 had no effect on homicide, rape, aggravated assault, robbery, or burglary. Larceny and motor vehicle thefts, however, seem to have increased moderately after Prop 47, but these results were both sensitive to alternative specifications of our synthetic control group and small enough that placebo testing cannot rule out spuriousness. Policy Implications As the United States engages in renewed debates regarding the scale and cost of its incarcerated population, California stands at the forefront of criminal justice reform. Although California reduced its prison population by 13,000 through Prop 47, critics argue anecdotally that the measure is responsible for recent crime upticks across the state. We find little empirical support for these claims. Thus, our findings suggest that California can downsize its prisons and jails without compromising public safety. - 'Criminology &Public Policy, Volume 17, Issue 3, Page 693-715, August 2018. '