Over the past 10 years, the healthcare system has undergone a significant shift in the structure of health care delivery. Many hospitals and physician groups have organized themselves to address the most pressing national priorities in health care: controlling increases in the cost of care and improving the quality of care that US residents receive. Payers, both public and private, have supported this shift by implementing new alternative models of payment that incent the delivery of cost-efficient,high-quality care. The most promising alternative payment model to fee-for-service payment is the accountable care organization (ACO)model. Some available evidence suggests that ACOs, both for Medicare beneficiaries and for commercially insured patients, reduce total cost of care and improve quality.1-3Although there are different ways to construct ACO models, at its core, an ACO is a contract between clinicians and the payer to meet rigorous clinical quality and experience goals and lower spending