This thesis presents the hydrogeological characterization of basement Aquifers in the Assosa area, Benishangul-Gumuz Regional State, Western Ethiopia. The study area covers 863 Sqkm of Hoha and Selga rivers and is mostly covered by crystalline basement rocks (hard rocks) which are regionally known to be called aquicludes. Recent well drilling in the area shows that the well yield varies significantly (from abandoned to high discharge well). Currently, the water demand in the area is highly rising. However, limited groundwater investigation in the area causes well abandonment. To avoid the problem, combined geological, geophysical (electrical resistivity and magnetic methods), and pumping test methods, were carried out to investigate the hydrogeological system of the area. The study aims to characterize the expected aquifer zone / water-bearing units and map surface and buried geologic structures and weak zones. Twenty-one (21) VES data points and seven (7) 2D ERI survey data points with Schlumberger and Wenner configuration were conducted along Hoha and Selga river structures identified by satellite gravity and 364 magnetic data. The magnetic data and gravity data were processed using Oasis montaji 6.4.2 whereas IPI2win and Surfer were used for preparing pseudo-depth,geoelectric sections, and 2D ERI. According to this research work, the hydrogeological set up of the area is highly heterogeneous and mainly controlled by geological structures, weathering, and fracturing intensity.The aquifer is both intergranular and weathered / fractured volcanic and basement aquifer with secondary porosity. The study area has a localized shallow and deep confined and unconfined aquifer nature. Accordingly, the aquifer area is highly heterogeneous and mainly controlled by geological structures, weathering, and fracturing intensity for hard rock aquifers, however, granular porosity is dominant in the shallow alluvial aquifers. The transmissivity varies from 3.94 to 76.6m2/d, hydraulic conductivity ranges from 0.17 to 2.24 m/d. Secondary porosity and permeability that develop from secondary processes mainly control the aquifer productivity of the area.