Mental health problems are among the most common problems encountered by primary care providers. Half of the care for common mental disorders in the United States is delivered in general medical settings. Primary care providers commonly diagnose and manage conditions such as dysthymia, major depressive disorder, problem drinking, and anxiety disorders. Multiple challenges exist in delivering high-quality mental health care in primary care settings. The quality of the care delivered in the primary care setting may vary: providers may have difficulty making referrals when needed to mental health professionals; the supply of mental health professionals may be inadequate in some areas; and patients may be reluctant to see a second provider. Patients with serious and persistent mental illness such as schizophrenia are often seen predominantly in specialty mental health settings yet often have substantial unmet general health needs including obesity, diabetes, and cardiac risk factors, sometimes exacerbated by medication treatment of their mental illness. "Integrated care" is one approach to addressing these currently unmet needs. In integrated care models, there is systematic linkage of mental health and primary care providers requiring communication or coordination between providers to meet both the mental and general health needs of the patients. The exact nature of the consultation and collaboration varies greatly across models, and may include telephonic or information technology. Often, a mental health professional is placed, permanently or intermittently, in the primary care practice. A key difference is that the integration implies a much closer and more coordinated system of care than prior consultation or referral models. Also key is the involvement of a second health care professional, which distinguishes integrated care from interventions that train primary care providers to treat patients with mental health conditions without the involvement of a mental health professional. In 2008, the Agency for Healthcare Research and Quality (AHRQ), working with the University of Minnesota Evidence-based Practice Center (EPC), conducted a systematic review of the literature evaluating the integration of mental health and substance abuse treatment with primary care. The review addressed six key questions. The Minnesota EPC authors found 33 trials examining the impact of integrating mental health specialists into primary care; 26 trials addressed depression. The studies reported positive results for symptom severity, treatment response, and achievement of remission when compared with usual care. The level of care integration did not seem to be related to treatment outcomes. The EPC authors also reported that the level of integration did not appear to be related to outcomes. Most of the studies addressed the integration of mental health professionals into primary care; few examined the integration of primary care into mental health. A majority of the studies have involved older patients, and some positive studies, having found improved outcomes with integrated care, have been largely composed of minority populations. The main barriers identified to a broader use of integrated care include programmatic costs, insurance coverage, and relationships with multiple payers. The VA was felt to offer a good model of a sustained program. Key elements of successful models included active support at all levels of the organization and specific funding.