Researchers first examined current sentencing practice in Pennsylvania to determine how actual sentences compared to sentences specified in legislative proposals. They then developed a methodology to provide, for any sentencing proposal, estimates of the impacts of proposals in terms of: (1) total number of offenders sent to State prisons; (2) types of offenders sent to State prisons; (3) costs of housing the resulting State prison populations. Analysis showed that mandatory minimum sentencing legislation could have very large and costly impacts on the State prison system. These impacts would not result from the imposition of stiff penalties for the most serious offenses, but from the commitment to State prisons of large numbers of the least serious offenders. Furthermore, judges who found the mandatory minimum sentences too stringent might well respond to the sentencing mandates in ways (such as acquittal) that would defeat the intent of the legislation. Therefore, in view of the substantial problems associated with mandatory minimum sentences, alternative ways of improving sentencing practices should be developed. Footnotes, appendixes, tables, figures