There are a large number of reservoirs in Wales reflecting the need for the supply of water for both industry and for consumption, both within the country itself and in neighbouring England. A number also provide hydroelectricity and many old reservoirs also provided motive power for industries, especially for the processing of minerals such as metal ores and slate.

Wales has a relatively high rainfall and significant areas of mountainous topography which are sparsely populated which ensures that water is a plentiful natural resource. Much of the south and east of England has significantly lower rainfall and a high population density which puts significant strain on the water supply infrastructure there. Collecting water in deep valleys with high river flows is far more efficient that trying to collect sparse rainfall in a generally flat landscape. Because of this engineers have viewed Wales as a potential supply of water. The rapidly growing populations of the industrial centres of Britain in the Industrial Revolution made the need for high volume supplies of clean water even more important and Wales was seen as the ideal source. Such engineering and economic views paid scant regard to the social, cultural and linguistic impacts that the constructions of large reservoirs in Wales would create.

Some reservoirs have been formed by the raising of dams on water courses, others are natural water bodies, the levels of which have been raised to various degrees both for direct supply of water or else for regulation of supply with water being extracted from the river some way downstream. Many reservoirs which were constructed for one purpose e.g. industry, may now serve different or additional purposes e.g amenity or, as in many cases, have been abandoned entirely. Reservoirs range in size with many under 1 acre in area serving local needs, up to Llyn Trawsfynydd which extends to 1,128 acres (456 ha). Llyn Tegid (Bala Lake) is a natural lake which at 1,192 acres (482 ha), slightly exceeds Llyn Trawsfynydd in size, but whose height has been slightly raised.

Within Snowdonia, numerous existing natural lakes were co-opted to provide local water supplies during the late 19th and early 20th centuries. Within the Carneddau, Llyn Dulyn and Llyn Melynllyn were dammed in 1881 and 1887 respectively, to provide water to Llandudno. Llyn Anafon was dammed in 1931 to allow water supply for Abergwyngregyn, Llanfairfechan and the surrounding areas.

Cardiff Corporation Waterworks opened both Llanishen Reservoir and Lisvane Reservoir in 1886. It later commissioned the construction of three reservoirs in Cwm Taf to supply water to the capital. Beacons Reservoir was the first to take shape, between 1893 and 1897, Cantref Reservoir was built between 1886 and 1892 and the damming of the largest of the three, Llwyn-onn Reservoir began in 1911 with the supply of water starting in the mid 1920s.

The construction of Crai Reservoir was completed in 1907 and supplied Swansea with water. Llandegfedd Reservoir was opened by the Newport Corporation in May 1965, covering 434 acres (1.76 square kms). It was originally built to supply water to Spencer Steelworks in Llanwern.

Grwyne Fawr Reservoir, the only reservoir within the Black Mountains massif was completed by Abertillery and District Water Board in 1928, serving to supply water to various towns in the traditional county of Monmouthshire. A holding reservoir in Cwmtillery was constructed as a part of the scheme. Blaenycwm Reservoir to the northwest of Brynmawr, was originally constructed as a water supply for Nantyglo Ironworks but enlarged for public supply during the 20th century.

Llyn Brianne was constructed in the late 1960s to provide additional water supply to South Wales via the Felindre water treatment works. The original intention was to supplement supplies of water to Swansea, Neath and Port Talbot.