Accountable care organizations (ACOs) are often discussed and promoted as driven by physicians, hospitals, and other health care providers. However, because of the flexible nature of ACO contracts, management organizations may also become partners in ACOs.We used data from 2013-15 on 276 ACOs from the National Survey of Accountable Care Organizations to understand the prevalence of nonprovider management partners' involvement in ACOs, the services these partners provide, and the structure of ACOs that have such partners. We found that 37 percent of ACOs reported having a management partner, and two-thirds of these ACOs reported that the partner shared in the financial risks or rewards. Among ACOs with partners, 94 percent had data services provided by the partner, 87 percent received administrative services, 68 percent received educational services, and 66 percent received care coordination services. Half received all four of these services from their partner. ACOs with partners were more heavily primary care than other ACOs. ACOs with and without partners had similar performance on costs and quality in Medicare ACO programs. Our findings suggest that management partners play a central role in many ACOs, perhaps supplying smaller and physician-run ACOs with services or expertise perceived as necessary for ACO success.