This Health Affairs brief provides an overview of accountable care organizations (ACOs), which are networks of physicians and other providers that agree to be held accountable for the cost and quality of the full continuum of care delivered to a group of patients. The brief covers the origins of the ACO concept, describes what makes ACOs different from existing health plans and provider arrangements, and summarizes the current status of adoption by Medicare and private health insurance plans. It also notes that based on the results of a five-year demonstration, ACOs will likely be able to improve clinical care quality but may have a harder time generating meaningful savings.