I test how the U.S. Sentencing Guidelines, promulgated by the United States Sentencing Commission, affected district court decision making from 1999 to 2003. Policymakers effectively used legal regulation to constrain judicial discretion by specifying (a) the sentencing ranges applicable to certain factual patterns, (b) the departures from these ranges allowed for prosecutors, or (c) the ... [Show full abstract] departures allowed for judges. My results establish significant differences in how judges make decisions depending on whether or not they were constrained by guideline ranges. When the guidelines curtailed judges’ discretion, judges followed these guidelines but overwhelmingly sentenced defendants to the minimum sentence possible under the guidelines. Conversely, when guidelines allowed judges to depart from the sentencing ranges and instead to exercise broad discretion, they used this authority, and case sentences, although widely varying, were right censored at a point at or below the guideline minimum. Finally, the lowest sentences arose from prosecutors’ motions rather than judges’ own initiatives.