In 2018, insurers exited from many Affordable Care Act (ACA) nongroup insurance marketplaces, and remaining insurers increased their premiums sharply in many areas. In 2019, most areas experienced modest increases, or even decreases, in premiums,1 despite the pending elimination of the individual mandate penalties. And nationally, more insurers entered marketplaces than exited.2 To better understand these developments, we conducted case study interviews with marketplace administrators and insurers selling marketplace coverage in ten states: California, Florida, Georgia, Indiana, Maryland, Minnesota, Ohio, Virginia, Washington, and West Virginia. These states vary considerably in the number of insurers participating in their marketplaces, as well as by changes in premiums in 2018 and 2019.