This research evaluates the impact of electronic monitoring as an alternative to prison on reoffending. Leveraging plausibly exogenous variation in sentencing outcomes generated by quasi- random assignment of judges, we find electronic monitoring reduces reoffending within 24 months by 16 percentage points compared to serving a prison sentence. For offenders who are less than 30, the reduction is 43 percentage points, with sizeable and significant reductions in reoffending persisting for 8 years. Our calculations suggest that criminal justice costs are reduced by around $30,000 for each eligible offender who serves their sentence under electronic monitoring rather than in prison.