The past several years have seen a surge of interest in using risk assessment in criminal sentencing, both to reduce recidivism by incapacitating or treating high-risk offenders and to reduce prison populations by diverting low-risk offenders from prison. We begin by sketching jurisprudential theories of sentencing, distinguishing those that rely on risk assessment from those that preclude it. We then characterize and illustrate the varying roles that risk assessment may play in the sentencing process. We clarify questions regarding the various meanings of "risk" in sentencing and the appropriate time to assess the risk of convicted offenders. We conclude by addressing four principal problems confronting risk assessment in sentencing: conflating risk and blame, barring individual inferences based on group data, failing adequately to distinguish risk assessment from risk reduction, and ignoring whether, and if so, how, the use of risk assessment in sentencing affects racial and economic disparities in imprisonment.