One of the key mechanisms for expanding health insurance coverage under the Affordable Care Act is the creation of new insurance exchanges–marketplaces where people can compare and purchase qualified private health plans based on benefits, quality, and price. Although one goal is to stimulate competition among private health plans, in most states the insurance markets for individuals and small businesses are highly concentrated. For example, in 30 states a single insurance company accounts for more than half the enrollees in the individual market, and in most states one or two insurers dominate the small-group market