The available literature is clear on several points: (1) that psychiatric problems are common in primary care,^{69 and 80}(2) that patients with psychiatric and substance use disorders have high utilization rates of primary medical services,^{37, 38, 70, 71, 86 and 91} (3) that psychiatric patients often are underdiagnosed and undertreated,^{17 and 36} and (4) that deliberate misdiagnosis may occur.⁷⁴ The impairment of health, well-being, and occupational and social functioning associated with psychiatric disturbances exceeds the impairments associated with most common medical illnesses.^{6, 93, 107, 112 and 114} Psychiatric disorders, particularly depressive disorders, have a negative impact on the prognosis of comorbid medical illness, including coronary artery disease,^{24 and 32} myocardial infarction,^{26 and 27} stroke,⁶¹ Parkinson's disease,^{16 and 96} diabetes,^{49 and 57} and some aspects of cancer treatment.^{59 and 93} The cost of untreated psychiatric illness is high and influences negatively the cost of treatment for other medical disorders.^{33, 34, 38, 66, 75 and 88} It is crucial that delivery of primary care services be enhanced to include improved models of psychiatric management.

Most of the literature concerning psychiatric treatment in primary care is concerned with providing care for patients with depressive and anxiety disorders. Limited access to medical care for patients with chronic psychiatric disorders, particularly schizophrenia, is another important issue that is less well represented in the literature.^{11, 13 and 67} A high proportion of these patients are served in community mental health centers that, owing to a variety of ideologic, political, and economic reasons, operate under a social services delivery model, with less emphasis on the medical problems of the patients. The positive and negative symptoms of the patients' illness combine with a typically high level of social disorganization to undermine attempts to obtain medical follow-up. Under the best of circumstances, access to medical care for the underinsured is difficult in many settings, and many psychiatric patients simply are not capable of negotiating the labyrinth of transportation, economic, and bureaucratic maneuvers involved in making a medical appointment. A full review of these issues is beyond the scope of this discussion. The reader is referred to selected references for review.^{29, 50, 64 and 102}

This article reviews current health care delivery models pertaining to the provision of integrated care, including a discussion of major health policy and intervention studies, followed by a discussion of future trends and areas needing further inquiry. The hierarchy of psychiatric interventions in primary care to be discussed is as follows:

1 Usual care by primary care provider

2 Usual care with psychiatric consultation

3 Development of primary care psychiatric screening instruments

4 Development of evidence-based treatment guidelines

5 Enhancing dissemination and implementation of guidelines

6 Development of collaborative models of care