Health care extension is an approach to providing external support to primary care practices with the aim of diffusing innovation. EvidenceNOW was launched to rapidly disseminate and implement evidence-based guidelines for cardiovascular preventive care in the primary care setting. Seven regional grantee cooperatives provided the foundational elements of health care extension-technological and quality improvement support, practice capacity building, and linking with community resources-to more than two hundred primary care practices in each region. This article describes how the cooperatives varied in their approaches to extension and provides early empirical evidence that health care extension is a feasible and potentially useful approach for providing quality improvement support to primary care practices. With investment, health care extension may be an effective platform for federal and state quality improvement efforts to create economies of scale and provide practices with more robust and coordinated support services.It is well established that robust primary care is essential to a high-value health care system. 1-3 However, the US primary care system faces considerable challenges. 4,5 Strategies are needed to assist practices in adapting to a rapidly changing health care landscape. 6,7 While there is no single solution to address these primary care needs, a program implemented in agriculture in the early 1900s that brought research and education directly to rural communities has been suggested as a model that could bring similar assistance to primary care. 8,9 The Cooperative Extension Service, established in 1914 under the Smith-Lever Act, was a cooperative undertaking of county, state, and federal partners. 10 The federal government funded each state's land-grant university to develop the workforce and infrastructure to spread agricultural expertise, create bidirectional communication between farmers and academic sites of knowledge production, and facilitate the diffusion of agricultural innovations. A key aspect of extension was the placement of extension agents in close proximity to communities to ensure consistent understanding of needs and delivery of services. 11 This effort introduced new technology and resources to farmers and contributed significantly to making food affordable and accessible. 12,13 Federal and state leaders have championed the use of the extension model to support and improve primary care. 1,8,9,13,14 Evidence shows that primary care practices benefit from external support to develop quality improvement infrastructure and expertise, 6 but strengthening the primary care infrastructure requires reaching many practices -small and large, rural and urban-across the nation to rapidly diffuse innovation and facilitate quality improvement. A cooperative extension for health care called the Primary Care Extension Program was included in the Affordable Care Act (ACA), although no funds were appropriated for it. This article presents a first look at early empirical findings from the...