Medicare Advantage (MA), the private option to traditional Medicare, now serves roughly 37 percent of beneficiaries. Congress intended MA plans to achieve efficiencies in the provision of health care that lead to savings for Medicare through managed competition among private health plans.Two elements are needed for savings to accrue: a sound payment policy and effective competition among the private plans. This brief examines the latter.We use data from 2009–17 to describe market structure in MA, including the insurers offering plans and enrollment in each U.S. county. We measure both actual and potential competitors for each county for each year.MA markets are highly concentrated and have become more concentrated since 2009. From 2009–17, 70 percent or more of enrollees were in highly concentrated markets, dominated by two or three insurers. Since the payment system used to reimburse insurers selling in the MA market relies on competition to spur efficiency and premiums that more closely reflect insurers' actual costs, these developments suggest that taxpayers and beneficiaries will overpay. We also find an average of six potential entrants into MA markets, which points to a source of competition that may be activated in MA. To tap into potential competition, further research is needed to understand the factors affecting entry into MA markets.