The literature indicates that offense seriousness and offender's prior record are the key determinants of sentences. Factors other than racial discrimination in sentencing account for most of the disproportionate representation of blacks in U.S. prisons, although racial discrimination in sentencing may play a more important role in some regions or jurisdictions, for some crime types, or in the decisions of individual participants. The evidence of discrimination on grounds of social and economic status is uncertain; the evidence on the role of sex in sentencing is only preliminary. The strongest and most persistently found effect of case processing variables is the role of guilty pleas in producing less severe sentences. Statistical models of past judicial sentencing practices are valuable but insufficient for formulating sentencing policy. The extent of compliance with reforms has varied as a function of several factors. The substantial increases in prison populations in jurisdictions that have adopted sentencing reforms continue preexisting trends in sentencing and do not appear to be substantially caused by these reforms. The validity of many impact studies is limited because of their failure to control adequately for changes in the mix of cases before and after the change takes effect. Responsible sentencing policy formulation requires baseline projections of the size and composition of prison populations with no policy changes, as well as estimates of the impact of various policy options. Analytical techniques for this purpose, although still crude, can estimate the effects of proposed policy changes and clarify the value choices. Footnotes and tables are supplied.