The hydrogeology of an aquifer within the dolomite/limestone formation that dominates the Gibraltar peninsula has been studied with a view to the possible development of contained groundwater resources taht occur as a thin lens of fresh water overlying sea water. Recharge rates (calculated by soil moisture and chloride balance) could be of the order of 400 000 m3 per year. This is significant in relation to the current demand for potable water of some 1 000 000 m3 per year, 900f which is provided by desalination. Exploration drilling focused on fracture zones, but tested vertical boreholes generally demonstrated a rapid decline in water quality, even where drawdowns were very small. Exploration also revealed oil pollution of the groundwater within the Rock, most significantly near the Naval Dockyards to the soutwest and less so beneath the North Face. The greatest potential for sustained discharge was found to occur in shallowly inclined boreholes. Discharge quality was clearly sensitive to a range of factors including immediate rainfall events, tidal fluctuations, vertical permeability of the aquifer, borehole design and anthropogenic effects. Further studies are needed to determine the significance of these controls and to assess whether development of the likely resources is practicable.