Water supply and sanitation in Gibraltar have been major concerns for its inhabitants throughout its history, from medieval times to the present day. The climate of Gibraltar is a mild Mediterranean one with warm dry summers and cool wet winters. Since daily records of rainfall began in 1790, annual rainfall has averaged a mean of 838 millimetres (33.0 in) with the highest volumes in December and the lowest in July. However, most of Gibraltar's small land area is occupied by the Rock of Gibraltar, a limestone outcrop that is riddled with caves and crevices. There are no rivers, streams, or large bodies of water on the peninsula, which is connected to Spain via a narrow sandy isthmus.

Nun's Well is an ancient underground water reservoir in the British Overseas Territory of Gibraltar. It is located at Europa Point, and is thought to be of the Moorish period. It represents some of the earliest evidence of an artificial water supply in Gibraltar. The name of the cistern is thought to be derived from the nuns associated with the Shrine of Our Lady of Europe. In the eighteenth century, Nun's Well supplied the military with water. In the early nineteenth century, it provided water for the brewery that was built next door. In 1988, the Royal Engineers constructed what is now the main building, which has a castle-like appearance. Nun's Well became the focus of controversy during the 2010-2011 restoration of the site.