Groundwater investigations undertaken in the Willunga Basin since 1974, which have included the drilling of stratigraphic wells, geophysical well logging, water level and salinity monitoring and surface water hydrology studies, have enabled the geological framework of the basin and the lithofacies variations within the Tertiary sediments to be more clearly understood. The spatial distribution of the aquifer systems and the variations in water quality within them have been defined, thus establishing the framework for a basic groundwater computer model. Groundwater monitoring has revealed declining water levels locally, but there is no evidence of serious depletion of the groundwater resources. Water balance estimates suggest an average annual safe groundwater yield of the basin of approximately 4000 Ml. Water of irrigation quality (less than 2000 mg/l total dissolved salts) is obtainable throughout most of the Willunga Basin, with the exception of the south-western area.