The tectonically quiescent Proterozoic (Texas craton) and Paleozoic (Gulf Coast) basement of Texas shows large contrasts in hydrogeological behavior, from being a fresh-water aquifer, where it crops out, to forming the base of several large sedimentary basins at depths >7 km (>23,000 ft). Little is known about the basement, such as its petrographic distribution and structural features. This chapter proposes a preliminary conceptual model of flow and defines five regional or supraregional flow systems of the Texas basement: far West Texas and Llano Uplift outcrops on the one hand and, from west to east, Basin and Range, Texas craton, Ouachita Fold Belt, and Gulf Coast Basin. The Texas craton forms a large part of the Texas basement, and some of the large faults impacting it coincide with the maximum horizontal stress orientation. This coincidence suggests a dominant, mostly topography driven flow system to the southeast, toward the Ouachita Fold Belt, which may act as a discharge zone. The craton has low geothermal gradients, implying mobile water deep into the upper continental crust but with extremely long flow paths (105 to 107 years). Paleozoic cover of the Texas craton begins mostly with the extensive, normally pressured, and permeable carbonate formations of the Ellenburger Group. The basement top is hydraulically connected to the sedimentary cover, which is used as a guide for basement flow and pore water characteristics. Total dissolved solids of the shallow basement pore water is probably relatively low (<150 g/L); patches of high total dissolved solids derive from Permian evaporative brines. These Na-Ca-Cl brines are likely more common and have higher concentrations in the deep basement stabilizing the density-stratified system. Hydraulic behavior of the Paleozoic basement east and south of the fold belt underlying the thick Mesozoic and Cenozoic Gulf Coast sediments is unknown, but we hypothesize that flow is updip toward the Ouachita Fold Belt.