Nationally, care delivery organizations are developing accountable care organizations (ACOs), but few have an appreciation of the importance of behavioral health services or knowledge about how to include them in an ACO since their funding and delivery are currently segregated from other medical services. This commentary reviews data on the impact of patients with concurrent medical and behavioral health conditions. They indicate that three-fourths of patients with behavioral health disorders are seen in the medical setting, but are largely untreated because few medical patients choose to access the behavioral health sector, which is where behavioral health providers are paid to work. Untreated behavioral health conditions in medical patients are associated with persistent medical illness and significantly increased total medical healthcare service use and cost, especially in those with chronic medical conditions. At a national level, those with behavioral health conditions use one-third of total healthcare resources. This will not change unless at-risk ACOs can effectively correct the mismatch between behavioral health patients and behavioral healthcare delivery. The authors suggest that ACO subcontracting for traditional segregated behavioral health services, whether from local provider groups or external vendors, will not achieve ACO-mandated access, treatment, and cost reduction goals. Rather, behavioral health specialists will need to become core ACO member providers. This will allow them to be deployed along with other member providers using value-added delivery approaches in the medical setting to integrate medical and behavioral health service delivery, and to achieve synergistic health and cost improvement.