Competition among private Medicare Advantage (MA) plans is seen by some as leading to lower premiums and expanded benefits. But how much competition exists in MA markets? Using a standard measure of market competition, our analysis finds that 97 percent of markets in U.S. counties are highly concentrated and therefore lacking in significant MA plan competition. Competition is considerably lower in rural counties than in urban ones. Even among the 100 counties with the greatest numbers of Medicare beneficiaries, 81 percent do not have competitive MA markets. Market power is concentrated among three nationwide insurance organizations in nearly two-thirds of those 100 counties.