The ACA significantly expanded health insurance coverage, particularly by broadening Medicaid eligibility and establishing health insurance marketplaces.

Before the ACA, Medicaid eligibility was often restricted to very specific low-income groups, primarily pregnant women, children, and individuals with disabilities. The ACA expanded Medicaid eligibility to include nearly all non-elderly adults with incomes up to 138% of the federal poverty level. This provision was initially a mandatory part of the law, but a 2012 Supreme Court ruling made it optional for states, leading to a patchwork of expansion across the country. Concurrently, the law created Health Insurance Marketplaces (or exchanges) where individuals and small businesses could shop for private health insurance plans. These marketplaces allowed consumers to compare plans, often with financial assistance (subsidies) based on income, making coverage more affordable for millions who didn't receive insurance through an employer or government program. These mechanisms were central to the ACA's goal of reducing the number of uninsured Americans.