The northern Wiso Basin lies in the central part of the Northern Territory between the Barkly Tableland to the east and the Victoria River district to the west and northwest. Its northern and northeastern limits are the upper reaches of the Daly River system and the Gulf of Carpentaria drainage system. The southern environs (which are not discussed in this Bulletin) are in the semi-desert country between the Stuart Highway in the east and Tanami and The Granites in the west. The region is arid to semi-arid, but nevertheless good grazing lands occur in the eastern, northern, and western parts, and both cattle stations and small townships have been established. Because of seasonal and sparse rainfall the surface water resources of the region are meagre, and both pastoral and community development are largely dependent on the availability of groundwater of reasonable quality. This study examines the data from over 500 water bores and the regional geological mapping, supplemented by scouthole drilling and 176 chemical analyses of groundwater from selected bores, in an attempt to relate the groundwater regime to the geological environment. The region comprises three Palaeozoic basins of mainly carbonate rocks and their surrounding Lower Cambrian and Precambrian basement rocks. From a groundwater point of view the most important of the basement rocks are Lower Cambrian volcanics, which crop out in the west and extend subsurface considerable distances eastward. Most of the groundwater is obtained from the carbonate sequences. Over large parts of the region these sequences are overlain by flat-lying Lower Cretaceous rocks which have a profound effect on the groundwater regime. Geomorphology also is important to the groundwater regime. The region is a gently undulating plateau which slopes northward and is bounded by the dissected country of the main coastal river systems; depth to economic quantities of water is governed by the depth below the surface of suitable aquifers, and throughout the plateau these are generally deeper than on its dissected margins. The main aquifers occur in volcanic and carbonate sequences, and about the dissected margins the groundwater regimes of both are very similar because they are both relatively close to recharge zones. The salinity of the groundwater in these situations is low. Beneath the Lower Cretaceous rocks and in the plains country in the southeast, many bores in the carbonate rocks tap groundwater which has travelled considerable distances from recharge zones and accordingly is saline. The availability of groundwater varies through the region, depending on the principal geological control. Water is stored in the volcanics in joint and fracture zones and the supplies are related to their incidence below the watertable in the borehole intersection, and the presence of recharge paths to them. Recent work has shown aquifers to be stratigraphically controlled overlapping flows and sedimentary interbeds. Additional work on these aspects may minimize unsuccessful drilling. The water in the carbonate rocks is stored in cavities and joints, considerably enlarged because of the solubility of the rock, and hence providing greater storage and supply than the volcanics. Furthermore, recharge is probably more effective because of open sinkholes and joints in karst topography. The Lower Cretaceous rocks are generally poor aquifers. Although the sequence contains a basal sandstone of good aquifer characteristics beneath impermeable claystone units, it is generally above the potentiometric surface and stores little water. Bores begun in the Cretaceous rocks commonly need to be drilled over a hundred metres to reach the more favourable underlying carbonate formations. Pollution may be a hazard at settlements where local recharge to groundwater occurs, particularly in the areas of karst carbonate outcrops.