The Affordable Care Act’s health insurance marketplaces rely on robust competition to control costs and to provide consumer choice. But the decisions of several large insurers to scale back their 2017 marketplace participation, and the failure of many health insurance co-ops will leave marketplace shoppers in many states with fewer choices than they had in 2016. Furthermore, those insurers remaining in the exchanges have often found their marketplace customers to be less healthy than they projected, and they are raising premiums in response. Our briefing focuses on these trends, what they mean for the long-term viability of the marketplaces, and what public policy steps can be taken to bring more healthy people into the risk pool and to encourage insurer participation in the individual market.