MassHealth introduced accountable care organizations (ACOs) for many of its members in March 2018. An ACO is a group of doctors, hospitals, and other health care providers that work together with the goals of delivering better care to members, improving the population’s health, and controlling costs. This brief serves as a resource for stakeholders (i.e., providers, health care administrators, policymakers) to help explain the key elements of MassHealth ACOs. A variety of topics are covered in this brief, including: member eligibility; ACO types; savings and losses; member enrollment and assignment; plan selection periods and fixed enrollment; primary care provider exclusivity; and ACO features, such as community partners.