Presents characteristics contributing to the formation of four accountable care organizations (ACOs) that serve rural Medicare beneficiaries, one each of the four census regions (West, Midwest, Northeast, and South). Semi-structured interviews were conducted on-site with ACO leaders and representatives of key stakeholder groups (e.g., board members, physicians, information technology managers). Four organizational characteristics emerged as influential in the formation of these ACOs. First is previous organizational integration experience, which includes physician-hospital organizations, independent practice associations and mergers. Second is experience in risk-sharing arrangements, which includes participation in the Medicare Advantage program and insurance plan ownership. Third are information technologies, especially shared electronic health records, which enable several ACO capabilities. And fourth is developing partnerships with health and human services organizations in local and regional communities. These findings can help rural providers interested in forming or participating in an ACO assess the status and potential gaps of their core structures and capabilities.