Groundwater supplies are experiencing a critical shortage in arid and semiarid regions, as population growth, and hence demand, is not matched by the development of new water supplies. Groundwater is often the only water source in these areas, making the preservation of groundwater quality a critical issue. Powerful tools for groundwater protection are vulnerability maps, which can be used to identify protection zones. A number of factors influence aquifer vulnerability, including geology, geography, land use, and commercial and industrial development. These factors often display a complex spatial pattern easily addressed with GIS. Each factor is entered in the GIS as a layer, and these layers can be superimposed in various combinations to derive the vulnerability maps. The use of GIS is illustrated with vulnerability maps of Gunyuzu basin, located in the semiarid region of northwest Turkey. The hydrogeologic parameters were identified and assigned weights depending on their potential to influence groundwater quality and quantity. The combined weights were mapped to identify critical protection areas. The vulnerability maps of the Gunyuzu basin were reclassified into 5 class schemes, i.e. very high vulnerability rating area is 0.18%, high vulnerability rating area is 6.19%, moderate vulnerability rating area is 34.79%, low vulnerability rating area is 50.07%, and very low vulnerability rating area is 8.77%.