Despite efforts over the last 30 years to promote diversion from jail for individuals with serious mental illness who have engaged in criminal behavior, few jail diversion programs have been adequately studied. To guide development of jail diversion services and encourage empirical research on their effectiveness, the authors describe the overall concept of jail diversion and the basic operations of such a program. They also outline research issues in evaluating the effectiveness of jail diversion programs, including problems encountered in randomized field trials and quasiexperimental designs. Implications of jail diversion services for mental health professionals include learning how to collaborate with law enforcement personnel, sufficiently integrating mental health and substance abuse services into the criminal justice system despite segregated funding streams, and ensuring that clients who are intensively monitored are also provided with adequate treatment to avoid jail recidivism.