The antitrust laws stand to protect consumers of health care services from conduct that would raise prices, lower quality, and decrease innovation by lessening competition. Importantly, though, vigorous antitrust enforcement does not impede accountable care organizations (ACOs) and similar collaborations that advance these same goals of better and more efficient care; in fact, by fostering competitive markets, the antitrust laws encourage such initiatives. This article summarizes the legal framework that the federal antitrust agencies — the Federal Trade Commission and the Antitrust Division of the US Department of Justice — use to analyze ACOs and other collaborations among health care providers. It outlines the guidance provided by the federal antitrust agencies concerning when ACOs and other provider collaborations likely would harm competition and consumers. In addition, it reviews common antitrust issues that can arise with ACOs and provides examples of enforcement actions that have prevented health care providers from taking or continuing anticompetitive actions.