In this brief, we examine the changes in the rates of uninsurance among Texans ages 18-64 from September 2013, before the opening of the first open enrollment period of the Affordable Care Act's (ACA) Health Insurance Marketplace, and March 2016, after the close of the third open enrollment period. We also examine the demographic characteristics of the uninsured and the changes in rates of uninsurance among various demographic groups. The data show that from 2013 to 2016, the rate of uninsured Texans ages 18-64 dropped by nearly one-third (30% decrease). Of ten demographic sub-groups examined, all showed decreases in rates of uninsurance, ranging from 15% to 51%. The steepest percentage decreases were experienced by Texans ages 50-64 (51% decrease) and Texans with incomes between 139% and 399% of the federal poverty level (42% decrease). The largest percentage point decreases were experienced by Hispanics (11.9 percentage points) and Texans ages 50-64 (10.8 percentage points). We conclude that the ACA has contributed significantly to reducing the rate of uninsured Texans ages 18-64.