Using data from the Current Population Survey (CPS) from 2000 to 2012 and Health Reform Monitoring Survey (HRMS) data for 2013 and 2015, this article develops an approach to estimate the number of people who would have been uninsured in the absence of the Affordable Care Act (ACA) and isolates the change in coverage as of March 2015 that can be attributed to the ACA. We produce counterfactual forecasts of the number of uninsured absent the ACA for 9 age-income groups and compare these estimates with 2015 estimates based on HRMS relative coverage changes applied to CPS-based population estimates.