The concept of accountable care organizations (ACOs) has been set forth in recently enacted national health reform legislation as a strategy to address current shortcomings in the U.S. health care system. This paper focuses on implementation issues related to these organizations, building on some initial examples. We seek to clarify definitions and key principles, provide an update on implementation in the context of other reforms, and address emerging issues that will affect the organizations' success. Finally, building on the initial experience of several organizations that are implementing accountable care and complementary reforms, we propose a national strategy to identify and expand successful approaches to accountable care implementation.