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.