By 2010 even the most casual observer of mental health care recognizes that the criminal justice system is inundated with persons who have mental illnesses and is ill equipped to address their needs. Many would lay the blame on deinstitutionalization, but a hydraulic model of transfer from hospital to jail is probably far too simplistic. In the late 1930s Lionel Penrose sought empirical evidence of transinstitutionalization from the mental health to the criminal justice system. After studying 14 European countries, he posited that the proportion of individuals who required institutional care remained relatively constant over time, assuming that if people with a mental disorder could not access hospital beds they would land in prison. Penrose wrote, "As a general rule if the prison services are extensive, the asylum population is relatively small and the reverse also tends to be true."

Since then the belief that the criminal justice and mental health systems are functionally interdependent has gained widespread acceptance—despite the lack of consistent empirical support for Penrose's hypotheses. Psychiatric hospital beds are disappearing and jails are growing, but many other causal factors have been implicated: the paucity of affordable housing, shortfalls in social insurance programs, inadequate community rehabilitative programs, and the limited number of supported employment programs—to name just a few.

Whatever the causes of the rising prevalence of individuals with mental illness in the criminal justice system, there is a clear need to develop and refine interventions at multiple entry points in the system. Several articles in this issue of Psychiatric Services describe and evaluate a range of diversion and treatment options that point the way to more appropriate and improved services at the interface. As a group they reinforce several take-home messages: Many persons with mental illness can and should be diverted from the criminal justice system, and there is likely no single point of diversion or program configuration to serve all people and local systems. However, it is clear that the deeper the person with mental illness penetrates the criminal justice system, or the longer the tenure there, the greater the peril. Those who cannot be diverted need better treatment than they are often afforded, although coordinating treatment across mental health and criminal justice interfaces can be daunting.

Finally, although the Penrose hypothesis may have led many to believe that persons with severe mental illness in either system have identical treatment needs, often they do not. Far too many mental health interventions have been haphazardly implemented for persons with a history of criminal justice involvement without consideration of the effects of the criminogenic environments that they come from or the developmental pathways that have spawned antisocial behavior. The reports in this issue illustrate the progress and the long road ahead in addressing the needs of persons with mental illness in jails and prisons.