This issue of JHPPL examines the current landscape of accountable care organizations (ACOs). An ACO is a network of hospitals, physicians, or other health care providers that share financial and medical responsibility for the coordinated care of a patient. Rare prior to the Affordable Care Act (ACA, or Obamacare) there are now approximately 700 ACOs in the United States. Their increased presence has sparked renewed debate about issues important to patients, providers, and taxpayers throughout the nation. Supporters of ACOs argue that they will streamline current inefficiencies in both care and payment systems, reducing unnecessary procedures and spending. Critics argue that ACOs encourage even greater consolidation among providers and hospitals, limiting competition and potentially driving up medical costs. The authors of this issue analyze what ACOs are expected to achieve in lowering costs and improving health care quality and look closely at how accountability and enforcement will work with ACOs under the ACA.