The promotion of accountable care organizations (ACOs), a new health care delivery and payment model designed to curb rising medical costs while improving quality, is one of the most important elements of the Affordable Care Act. The ACO model is based on shared-risk contracts, in which ACOs agree to share the financial risk of health care overspending with third-party payers. Although they originate in Medicare, these shared-risk arrangements are quickly spreading to the private insurance markets, where they aim to dismantle the volume-driven fee-for-service revenue model.1 Hundreds of health systems across the country have already adopted the ACO model and in so doing have taken on a new role of cost containment. What may be less clear to them is that they are taking on new liability risks.