A GIS Decision Support System has been developed for the evaluated of Renewable Energy Sources potential and the financial analysis of RE investments. A GIS database with data on wind, topography, urban areas, and special activities has been developed and used for the evaluation of theoretical potential through the spatially continuous mapping if Renewable Energy Resources. The available and technological potential are evaluated by the application of availability and technological restrictions. The evaluation of economical potential is perform by a precise estimation of the expected energy output and installation cost. The financial analysis based on the Internal Rate of Return, identifies the financial viability of alternative investments. The evaluation of wind energy potential for the island of Crete, Greece and the financial analysis of a wind park installation are present as a case study.