Jail diversion is a strategy used widely to address the growing overrepresentation of people with mental illness in the criminal justice system. Jail diversion programs identify offenders in the criminal justice system that have a mental illness and will be better served by appropriate community-based treatment rather than by incarceration. Not only does treatment reduce the individual’s chances of recidivism, but it also improves public safety by freeing beds in jails and prisons for serious and violent offenders whose crimes were not a result of a mental health problem. One aspect of jail diversion that has proven more difficult to analyze is its fiscal impact. In order to address this gap HSRI developed a model to quantify the potential cost offsets of jail diversion programs. The model is a computerized simulation that projects the effectiveness, costs, and potential cost savings of implementing jail/prison diversion programs for offenders with mental illness, the majority of whom have co-occurring substance abuse disorders. This cutting-edge, computerized tool helps policy makers and public officials consider different service package, resource allocation options and their fiscal implications.