The 'Rock' of Gibraltar is a small isthmus at the western end of the Mediterranean Sea overlooking the Straits of Gibraltar, the North African Coast and Spain. For the last 1,000 years Gibraltar has been a strategic military location guarding the gateway between the Mediterranean and the Atlantic Ocean. Water has always been in short supply on the 'Rock'. In olden times rainwater was collected, and earlier this century the Rock was partially covered by a concrete sheath to aid natural catchment. During the last 40 years rainwater has been supplemented by a variety of seawater desalination processes and the remnants of some of these early desalting plants can still be seen today. The 36,000 inhabitants, made up of Gibraltarians, tourists and military personnel have a daily requirement of 4,500m(3)/d of fresh water, supplied from several sea-water desalting plants owned by the Ministry of Defence (MOD) and the French water utility, Lyonnaise des Eaux. The main production centres are the 1,680m(3)/d seawater RO facility at Glen Rocky and the North Mole installation producing 2,400m(3)/d by MSF evaporation and 480m(3)/d by seawater RO. The remainder is supplied from an MED plant attached to the local refuse incinerator, with an output determined by the volume of refuse to be burned. This paper reviews the long-term operation of the seven 240m(3)/d seawater RO units at Glen Rocky. It discusses alterations and improvements made to chlorination, flocculation, scale control, and product water treatment. It details the way chemical costs have changed during the past 12 years without jeopardising output or quality. It also details increases in chemical costs due to implementing improvements associated with Health and Safety legislation. The paper also considers a few of the unforeseen problems the operator has experienced. Although Gibraltar is no longer a NATO base, fresh water is still a prime need for the population. Water supplies could be met by laying a pipeline fi-om Spain, bur the desire of every nation to be self-sufficient in water coupled with long-term political differences between Spain and Gibraltar precludes this option. Gibraltar has entered the new millennium as the only part of Europe with a population totally dependent on seawater desalination.