This brief provides considerable data on insurer participation and premiums in several markets in 8 study states. The brief concludes that plan participation is less a function of whether states have state-based (SBM) or federally-facilitated (FFM) health insurance marketplaces and more a function of the pre-Affordable Care Act (ACA) insurance market, as well as the managed competition framework in the ACA. In both SBM and FFM states there are a large number of participants in most markets, including large and small commercial insurers and some new entrants such as Medicaid plans and Co-Ops. In most markets, premiums are lower than expected, although in markets with few plans and little competition, the ACA has had much less impact.