Using the American Community Survey, this brief examines how participation and uninsurance changed among children who were eligible for Medicaid/CHIP between 2013 and 2014, the first year of implementation of the Affordable Care Act (ACA)’s major coverage provisions. Medicaid/CHIP participation among eligible children rose by 2.3 percentage points, from 88.7 percent in 2013 to 91.0 percent in 2014, and the number of eligible uninsured children fell from 3.5 million to 2.8 million. By 2014, Medicaid/CHIP participation rates were over 90 percent in 32 states, with larger gains in participation in states expanding Medicaid under the ACA.