Many observers expected enrollment in ACA insurance marketplaces to decline in 2018, compared to 2017, due to Congressional efforts to repeal and replace the law, funding cuts for marketplace advertising and outreach, and this year’s shortened open enrollment period. Yet 15 states actually saw higher enrollment this year (although the remainder did experience the expected declines). To understand why these dual trends emerged, the Urban Institute interviewed state officials, insurers, and other key players in five states (Rhode Island, Washington, New York, West Virginia, Louisiana). We found that shifts in marketplace enrollment reflected whether the cheapest premiums were available on- or off-marketplace, whether states maximized the size of premium tax credits through “silver loading,” the amount of marketplace advertising, the length of open enrollment periods (which some states did not shorten), and whether state officials encouraged consumers to enroll in coverage.