Dams and reservoirs are often considered as sustainable and as important contributions to the development of renewable energy sources. However, the lifetime of these facilities is generally limited to some decades or somewhat more than a century, and their exploitation may have an important impact on the environment. This paper reviews briefly reservoir processes, which are mainly lake processes, and environmental impact of reservoirs, such as (1) Impacts on the hydrological system and river dynamics, (2) impacts on physico-chemical cycles, (3) impacts on soil, surface and groundwater quality and environmental toxicity, (4) impacts on ecosystems and (5) the question of end-of-life planning. A concept for the improvement of planning and reservoir management with respect to sustainability is put forward.