The accountable care organization (ACO) model, like the HMO model, aims to promote provider accountability for care coordination. However, in an ACO, there is no gatekeeper primary care physician (PCP), and members can seek care from any provider they choose. In order to measure provider performance in such a system, members must be assigned to providers via an analysis of healthcare claims. Patient attribution methods that support valid and actionable cost and quality metrics will be crucial to the development of an effective ACO model.