This issue paper examines the latest developments in accountable care organizations (ACOs), including a look at the final regulations on ACOs issued in October 2011 by the Centers for Medicare & Medicaid Services (CMS). Written by the Urban Institutes Bob Berenson and Rachel Burton, this paper provides an overview of ACOs, the key complaints about CMS' proposed regulations and their resolution in the final regulations, and the status of adoption of this new model for delivering health care by both Medicare and private health insurance plans. This paper is supported by the Robert Wood Johnson Foundation.