This article provides an overview of the water reservoirs in the world. There are nearly 50 000 large dams in the world, retaining more than 6500 km 3 of water. Creation of reservoirs has inundated an area like the size of France or California and has forced 40–80 million people to resettle. The majority of these reservoirs have been built during the last 50 years, but new reservoirs are still constructed at a rate of nearly one new reservoir per day. Reservoirs are used for irrigation, hydroelectric power generation, domestic purposes, flood control, and recreation. The world's largest reservoir is Lake Volta in Ghana, retaining 148 km 3 of water. The world's largest dam is the 2309 m long at crest and 185 m high from crest to base. Three Gorges Dam in China. Reservoirs may show large fluctuations in water level, especially hydroelectric reservoirs in regions where large volumes of flood water are stored for use during dry periods. New reservoirs undergo ecological succession during which original communities are destroyed and new ones struggle to become established, coping with artificial environmental conditions. Reservoirs trap large amounts of sediments, causing coastal deltas to shrink, and they affect the nutrient balance in the sea. Some reservoirs also produce considerable amounts of greenhouse gases and speed up the dispersal of exotic species.