The Affordable Care Act (ACA) relies on insurers to offer health plans in the individual health insurance Marketplaces. Since the ACA's implementation, levels of Marketplace competition have varied, reaching a nadir in 2018. We examined the characteristics of counties that experienced changes in insurers' participation in the ACA Marketplaces from 2016 to 2021. Using data from the Kaiser Family Foundation and other sources, we found that 1,968 counties (accounting for 66 percent of the US population younger than age sixty-five) have more insurers in 2021 than in 2018, whereas only twelve counties (comprising 0.4 percent of the US nonelderly population) have fewer insurers. The number of counties with monopolist Marketplace insurers declined from 1,616 in 2018 to 294 in 2021. Recent Marketplace insurer gains were more likely in counties that lost insurers from 2016 to 2018 or had a monopolist insurer in 2018. Increased competition may lead to lower gross premiums in the ACA Marketplaces. Given the Biden administration's support for the ACA Marketplaces, it appears likely that the ACA individual health insurance market will be stable and profitable for the next several years.