The number of accountable care organizations (ACOs) increased rapidly during 2012. There are now more than 250. This increase is likely to accelerate: commercial health insurers are signing ACO-like contracts with health care organizations, and the return of President Obama to the White House, as well as the Supreme Court ruling upholding the Affordable Care Act (ACA), have removed uncertainty about the Medicare ACO program. The goals for ACOs are well known: to control health care costs, to drive quality in health care, and to improve population health.