Policy makers and analysts have been voicing concerns about the increasing concentration of health care providers and health insurers in markets nationwide, including the potential adverse effect on the cost and quality of health care. The Council of Economic Advisers recently expressed its concern about the lack of estimates of market concentration in many sectors of the US economy. To address this gap in health care, this study analyzed market concentration trends in the United States from 2010 to 2016 for hospitals, physician organizations, and health insurers. Hospital and physician organization markets became increasingly concentrated over this time period. Concentration among primary care physicians increased the most, partially because hospitals and health care systems acquired primary care physician organizations. In 2016, 90 percent of Metropolitan Statistical Areas (MSAs) were highly concentrated for hospitals, 65 percent for specialist physicians, 39 percent for primary care physicians, and 57 percent for insurers. Ninety-one percent of the 346 MSAs analyzed may have warranted concern and scrutiny because of their concentration levels in 2016 and changes in their concentrations since 2010. Public policies that enhance competition are needed, such as stricter enforcement of antitrust laws, reducing barriers to entry, and restricting anticompetitive behaviors.