This brief examines the conditions under which competition in health insurance exchanges is likely to be effective in placing downward pressure on insurance premiums. We conclude that areas with a single dominant insurer or a dominant hospital system are less likely to experience effective competition. In markets in which there are several insurers with significant market share and no dominant hospital system, the result could be limited or tiered network products that could successfully constrain the cost of premiums. Participation of existing Medicaid plans may also increase effective competition in health insurance exchanges.