In recent years, the concern about global warming, the acid rain and the harmful effects of fuel emissions, has created new demand for clean and sustainable energy sources, like the wind and the hydro power. Today, wind turbines and Small Hydroelectric plants are the most rapidly developing energy sources in the world, following increasingly rapid rhythms in the past few year. The environmental benefits, afforded by these clean and abundant energy sources, as well as their competitiveness, with regard to cost in the world energy markets, made wind and water the alternative energy sources of choice, fundamental to the energy policies of the technologically developed countries. The European Union has promoted all the Renewable Energy Sources (RES), by setting ambitious national targets for the member states with regard to the RES' s participation in the generation of electricity. Specifically for Greece, the electricity generation target from RES' s is 20,1%.