--- - |2+ Research Summary In this study, we use both a regression discontinuity design and an instrumental variable identification strategy to examine the relationship between prison length of stay and recidivism among a large sample of federal offenders. We capitalize on the U.S. Sentencing Guidelines structure to apply these strong inference, quasi‐experimental approaches. We find that average length of stay can be reduced by 7.5 months with a small impact on recidivism. We also examine whether there is treatment heterogeneity. We find that length‐of‐stay effects do not vary by criminal history, offense seriousness, sex, race, and education level. Policy Implications We show that reducing the average length of stay for the federal prison population by 7.5 months could save the Bureau of Prisons 33,203 beds once the inmate population reaches steady state. This back‐of‐the‐envelope estimate reveals how reductions in time served can have a much larger impact on prison reductions compared with diverting low‐level offenders from prison to probation. Prison length‐of‐stay reductions can impact the entire prison population, whereas diversion typically affects a small subset of offenders whose consumption of prison beds is a small fraction of the total number of beds. We also discuss the potential impact of reducing levels of imprisonment on other collateral consequences. - 'Criminology &Public Policy, Volume 17, Issue 3, Page 731-769, August 2018. '