Will accountable care organizations (ACOs) deliver high-quality care at lower costs? Or will their potential market power lead to higher prices and lower quality? ACOs appear in various forms and structures with financial and clinical integration at their core; however, the tools to assess their quality and the incentive structures that will determine their success are still evolving. Both market forces and regulatory structures will determine how these outcomes emerge. This introduction reviews the evidence presented in this special issue to tackle this thorny trade-off. In general the evidence is promising, but the full potential of ACOs to improve the health care delivery system is still uncertain. This introductory review concludes that the current consensus is to let ACOs grow, anticipating that they will make a contribution to improve our poor-quality and high-cost delivery system.