The geothermal use of the subsoil can be a solution for the reduction of CO2 emissions and other greenhouse gases.Geothermal Energy is renewable and absolutely sustainable.Especially for heating and cooling of buildings the investment in geothermal systems is not only technically possible and economically reasonable but also promising as there is a large saving potential in the energy consumption related to buildings.Geothermal facilities require a well-engineered dimensioning and design in order to be cost-effective on the one hand and technically reliable on the other had.Within projects extensive analyses have been conducted in order to investigate the performance of geothermal energy systems and their environmental impact.The authors show results of analyses which have been conducted in order to investigate the performance of geothermal energy systems and their environmental impact and introduce to project examples.