The Affordable Care Act (ACA) is credited with reducing the number of uninsured by 20 million and bringing the uninsurance rate to a historic low. In this brief, we take a detailed look at changes in health insurance coverage among the nonelderly population from 2010 to 2015. We use data from the American Community Survey to compute uninsured rates for the nation, individual states, and demographic groups. We find that gains in coverage were large and broad-based for all Americans under 65, for both men and women, and across subgroups based on race/ethnicity, levels of educational attainment, and states. Congress is now considering options to repeal the ACA; removal of key provisions without new policies capable of maintaining these coverage gains would result in millions of Americans, of all ages and backgrounds and in all states, losing health insurance.