Hydroelectricity is the most widely employed of all renewable energies. It refers to electricity generated from the gravitational force of falling or free-flowing water. It has been used since ancient times for grinding flour and the like in water mills, and first became used for electricity production following the discovery of electricity in the 1880s. As hydroelectric plants grew and dams begun to be constructed, the dams themselves also started becoming methods of flood control, irrigation and navigation.

Hydroelectric power is derived from the potential energy of dammed or free-flowing water driving a water turbine and generator. Large and small hydroelectric facilities tend to sport dams. They generally generate electricity for nationwide grids, though the latter type may also serve independent communities or industries in areas unconnected to a power network. Micro and pico hydros, which are smaller versions, tend to be run-of-the-river facilities without dams for single households or small communities in the developing world. The amount of electricity generated from any of these hydroelectric power plants depends on the volume of water that is involved and the distance in height that the water falls. A multiple reservoir facility may also serve as a means of supplying high peak demands. At low electrical demand water is simply pumped up to the higher reservoir, only to be released back through the turbine at a time of higher demand. This feature of hydroelectric dams is the most effective means of power grid energy storage there is.

The appeal with hydroelectric dams is how they do not rely on fuel for generating electricity. Indeed, they are thought to be the cleanest type of energy available in how they produce virtually no carbon dioxide when operated and only moderate such when constructed. Moreover they are cost effective in how they require very little operating labour and how they last longer than most other power plants, not to mention reliable in how they - unlike wind power or solar power - are a controllable source of energy that can be increased or decreased according to the needs of the power network.

However hydroelectric power plants are not without their drawbacks. Especially the large hydroelectric reservoirs spare the environment of greenhouse gas emissions only to put large areas of land upstream from the dam under water. Not only does this submerge healthy lowlands, but it also displaces both human and animal populations. The damage done to former residents of a hydroelectric dam is not one easily repaired by monetary compensation. Moreover dams are disruptive to the aquatic ecosystems of the river both upstream and downstream. Species of fish like salmon tend to suffer particularly badly, for which hydroelectric technologies that would go easier on the fish are currently being researched. Finally dams pose a significant risk, as should they somehow falter, break or be destroyed they would cause incredible damage to the population and infrastructure downriver. Indeed, some of the largest catastrophes in the history of mankind have been dam failures.

Hydroelectric power plants already make up a significant part of the world's power supply. It is not unusual for it to amount to 25 to 50 percent of a country's power production, and in the case of countries like Congo, Paraguay, Norway and Brazil it is responsible for over 85 percent of the national power supply. Yet if the recent mega-constructions like the three gorges dam of China are any indication there is no question of that it is an industry that will keep on growing still. Hopefully technological progress will bring about sustainable ways to solve or circumvent its many negative aspects, however impossible that might seem presently.