As the Centers for Medicare & Medicaid Services prepares to approve applications to become Medicare Shared Savings Program accountable care organizations (ACOs) beginning in April 2012 and Pioneer ACOs begin their operations, the Robert Wood Johnson Foundation and Health Affairs have published this policy brief describing the history, development, and implementation of the concept of accountable care organizations. The policy brief highlights both the potential as well as the remaining unresolved questions about the capacity of ACOs to achieve the improvements and reductions in costs that are intended, especially within the short timeframes being used in some of these first federal government models.