Individuals with alcohol and drug use problems may receive health care from medical, mental health, and substance abuse providers, or a combination of all three. Systems of care are often distinct and separate, and substantial opportunities for benefit to patient, provider, and payer are missed. In this article, we outline (1) the possible benefits of linking primary care, mental health, and substance abuse services from the perspective of the major stakeholders-medical and mental health providers, addiction clinicians, patients, and society-and (2) reasons for suboptimal linkage and opportunities for improving linkage within the current health care system. We also review published models of linked medical and substance abuse services. Given the potential benefits of creating tangible systems in which primary care, mental health, and substance abuse services are meaningfully linked, efforts to implement, examine, and measure the real impact should be a high priority.