There is a need to improve care at the interface of general medicine and mental health. Provision of care at this interface is the aim of integrated care. Integrated care occurs when mental health specialty and general medical care providers work together to address both the physical and mental health needs of their patients. This comprehensive systematic review addresses the evidence for integration of mental health services into primary care settings and primary services into specialty outpatient settings. The research questions were: 1) What models of integration have been used? a) What theoretical models support these programs? b) What is the evidence that integrated care leads to better outcomes? 2) To what extent does the impact of integrated care programs on outcomes vary for different populations (e.g., specific mental illness conditions, chronically ill, racial/ethnic groups, elderly/youth)? 3) What are the identified barriers to successful integration? a) How were barriers overcome? b) What are the barriers to sustainability? 4) To what extent did successful integration programs make use of health information technology (IT)? 5) What financial and/or reimbursement structure was employed in successful integration programs? Is there evidence to suggest that any specific financial/reimbursement strategy is superior to another? 6) What are the key elements of programs that have been successfully implemented and sustained in large health systems? To what extent do they follow, or how do they differ from, models that have been studied in published research studies? The scope of the review included alcohol addiction but not other forms of substance abuse. Inpatient settings are also excluded. The review focuses on four areas: (1) specifying what integration is (and is not); (2) detailing the process through which integrated care may affect clinical outcomes; (3) expanding beyond the scope of prior reviews to include multiple illnesses and patient populations; and (4) specifying the conditions under which various models of integrated care are likely (or unlikely) to work in 'real-world' settings. This review also conducted case studies in order to better understand the implementation of integrated care models.