Involvement with people with a mental illness (PMI) has been a feature of the role of police in Western societies since the earliest mental health legislation in the 19th century. It is only in recent times, however, and especially since the era commencing in the 1960s with the widespread closure of most asylums for the mentally ill, that this role has become the subject of systematic research and analysis. Early literature reflects concern about a role thrust on police as state-provided, institutionally based mental health services were rolled back. Police were confronted with a new phenomenon: the presence, especially in urban centers, of large numbers of people with disabling mental illness for whom there was no longer a residentially based mental health service, and frequently little in the way of a community-based program of mental health care and social services. Police were left to deal as best they could with what was often a crisis situation. One of the principal resulting consequences of a lack of comprehensive policy in this area, and the absence in particular of collaboration with and by mental health services, became a well-known cycle of arrest, incarceration, inadequate treatment, and chronicity for persons with mental illness. Without an effective model of interaction, any police response also ran the risk of escalating increasingly to the use of aversive measures, including deadly force. For these reasons, progressive police agencies sought new approaches to their involvement with people with mental illness. Since at least the 1960s the scholarly literature is reflective of this search. From this recognition emerged a variety of policing models designed to enhance the delivery of assistance to persons experiencing some form of mental health crisis. One of these models, the so-called Memphis Crisis Intervention Team (MCIT), has proven to be particularly influential and has now been widely adopted in some form or other in police agencies across North America and elsewhere. There is a considerable literature about the MCIT program that suggests it is effective in many areas, including changes in police beliefs and attitudes, improved pathways for those apprehended, and improved referrals to mental health services. There is less evidence about the effectiveness of other models as the literature reviewed in this article indicates. Attention is also directed to a number of emerging issues as well as those surrounding the education and training of police personnel, the protection of special populations de-escalation of crises, the use of force, and the particular challenges of extending these police response models to less developed parts of the globe.