Purpose: To provide empirical evidence on key organizing constructs shaping practical, real-world integration of behavior health and primary care to comprehensively address patients' medical, emotional, and behavioral health needs.Methods: In a comparative case study using an immersion-crystallization approach, a multidisciplinary team analyzed data from observations of practice operations, interviews, and surveys of practice members, and implementation diaries. Practices were drawn from 2 studies of practices attempting to integrate behavioral health and primary care: Advancing Care Together, a demonstration project of 11 practices located in Colorado, and the Integration Workforce Study, a study of 8 practices across the United States.Results: We identified 5 key organizing constructs influencing integration of primary care and behavioral health: 1) Integration REACH (the extent to which the integration program was delivered to the identified target population), 2) establishment of continuum of care pathways addressing the location of care across the range of patient's severity of illness, 3) approach to patient transitions: referrals or warm handoffs, 4) location of the integration workforce, and 5) participants' mental model for integration. These constructs intertwine within an organization's historic and social context to produce locally adapted approaches to integrating care. Contextual factors, particularly practice type, influenced whether specialty mental health and substance use services were colocated within an organization.Conclusion: Interaction among 5 organizing constructs and practice context produces diverse expressions of integrated care. These constructs provide a framework for understanding how primary care and behavioral health services can be integrated in routine practice. (J Am Board Fam Med 2015;28: S7-S20.)