This brief, compiled by experts from our Center for Health Law and Economics, goes over the basics of MassHealth's introduction of Accountable Care Organizations (ACOs). It includes a brief introduction of what an ACO is, the three distinct types of ACOs, who they serve, the services they provide and how they can improve care and contain costs. It also includes a list of resources for readers interested in learning more about this new approach to healthcare payment. Client/Partner: Blue Cross Blue Shield of Massachusetts Foundation