Thousands of years ago mankind learned how to exploit the mechanic energy produced by falling water. The Greeks and Romans already used water mills to grind wheat.Great technical progress was achieved following the evolution of the water wheel into the turbine, i.e. an equipment capable of transforming mechanic energy into electric energy. The creation of the hydraulic turbine dates back to the end of the 1800s. Since the earlier times, this technology has been improved further, and today the total yield of the more modern plants is over 80%. This means that if the energy of water is equal to 100, the useful energy supplied by a hydroelectric power plant is equal to 80.