Viewing primary care as merely one of the many niches in individual patient care fosters a narrow perspective that neither accounts for the full value of primary care nor offers proper guidance for reforming it. Instead, a systems view better articulates primary care's essential role in increasing the effectiveness and efficiency of other health care services. Primary care improves health care system functioning through such services as managing and triaging undifferentiated symptoms, matching patient needs to health care resources, and enhancing the system's ability to adapt to new circumstances. By acting as an appropriate filter for high-technology care, primary care helps ensure that it is appropriately applied, a major determinant of outcomes. Furthermore, because primary care is the most financially and geographically accessible arm of the health care system, it reduces socioeconomic and geographic disparities across the population. Even in the de facto health system that exists in the United States, these functions are necessary for the efficient, equitable, and sustainable delivery of health care and the adaptability and resilience of the system as a whole. As primary care reforms proceed, it will be important to explicitly consider how to support and strengthen its system-level benefits. Needed reforms include enabling universal access to primary care and providing the leadership, training, and infrastructure support to improve its performance as a central hub in the network of patients, clinicians, and communities. It is important that health systems realize the essential integrative function of primary care.