There are now more than seven hundred accountable care organizations (ACOs) in the United States. This article describes some of their most salient characteristics including the number and types of contracts involved, organizational structures, the scope of services offered, care management capabilities, and the development of a three-category taxonomy that can be used to target technical assistance efforts and to examine performance. The current evidence on the performance of ACOs is reviewed. Since California has the largest number of ACOs (N=67) and a history of providing care under risk-bearing contracts, some additional assessments of quality and patient experience are made between California ACOs and non-ACO provider organizations. Six key issues likely to affect future ACO growth and development are discussed, and some potential “diagnostic” indicators for assessing the likelihood of potential antitrust violations are presented.