Accountable care organizations (ACOs) and similar reforms aim to improve coordination between health care providers; however, due to the fragmented nature of the US health care system, successful cordination will hinge in large part on the ability of health care organizations to successfully partner across organizational boundaries. This article findings suggests that the success of the ACO model will hinge in large part upon the success of new partnerships, with important implications for understanding ACO readiness and capabilities, the relatively small savings achieved to date by ACO programs, and the path to providers bearing more risk for population health management.