The free renewable source of energy provided by falling water that drives the turbines. Hydropower is the most important of the regenerable energy sources because of its highest efficiency at the energy conversion. There are two types of hydroelectric power plants: a) run-of-river power plants for the use of affluent water; b) storage power plants (power stations with reservoir) where the influx can be regulated with the help of a reservoir. Mostly greater differences in altitudes are being used, like mountain creeks. Power stations with reservoirs are generally marked by barrages with earth fill dam or concrete dams. Though hydropower generally can be called environmentally acceptable, there exist also some problems: a) change of groundwater level and fill up of the river bed with rubble. b) Risk of dam breaks. c) Great demand for land space for the reservoir. d) Diminution, but partly also increase of value of recreation areas. As the hydropowers of the world are limited, the world energy demand however is rising, finally the share of hydropower will decrease.