With deinstitutionalization and the influx into the community of persons with severe mental illness, the police have become frontline professionals who manage these persons when they are in crisis. This article examines and comments on the issues raised by this phenomenon as it affects both the law enforcement and mental health systems. Two common-law principles provide the rationale for the police to take responsibility for persons with mental illness: their power and authority to protect the safety and welfare of the community, and their parens patriae obligations to protect individuals with disabilities. The police often fulfill the role of gatekeeper in deciding whether a person with mental illness who has come to their attention should enter the mental health system or the criminal justice system. Criminalization may result if this role is not performed appropriately. The authors describe a variety of mobile crisis teams composed of police, mental health professionals, or both. The need for police officers to have training in recognizing mental illness and knowing how to access mental health resources is emphasized. Collaboration between the law enforcement and mental health systems is crucial, and the very different areas of expertise of each should be recognized and should not be confused.