Abstract The 6 km 2 peninsula of Gibraltar is unusual hydrogeologically as, in effect, a small but high limestone island, subject to a Mediterranean climate of cool wet winters and warm dry summers. Provision of an adequate water supply for its town and garrison has been a continuing problem, particularly as the population has grown from about 3000 in the 18th century to over 30 000 by the end of the 20th. The narrow peninsula is dominated by the Rock, a mass of Lower Jurassic dolomite and limestone whose main ridge has peaks over 400 m high. Early supplies of potable water were from roof and slope rainwater runoff, and from shallow wells in the Quaternary sands that cover ‘shales’ flanking the Rock at low levels. Intermittent hydrogeological studies through the 19th and 20th centuries, notably in association with the British Geological Survey in 1876, 1943–1952, and 1974–1985 attempted to develop inferred groundwater resources within the sandy isthmus which links the Rock to southern Spain and in the Rock itself. Problems resulted from inadequate understanding of the geology, of recharge, of the behaviour of aquifers containing saline water at depth and of the need to protect aquifers from pollution. Failure to extract adequate groundwater led to development of a separate supply of saline sanitary water to reduce demand for potable water and innovative attempts to improve slope catchment of rainwater, before near-total commitment to desalination for potable supplies in 1993.