The Gulf Coast Regional Aquifer-System Analysis study includes all major Cenozoic aquifer systems in the Gulf Coastal Plain of Alabama, Arkansas, westernmost Florida, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and Texas. Although Cenozoic deposits are not uniformly differentiated, interstate correlations of major Paleocene and Eocene units are generally established, and the altitude of the top and thickness of seven units were mapped. Younger deposits are not as well differentiated; therefore, only three of the younger units were mapped.