The choice of a geothermal heat pump system, allows to use the ground as a heat reservoir with which to exchange heat during the year; this involves that the effect of seasonal temperature variations are minor than the variations of the outside air. In this book two air -conditioning systems for buildings are compared: a geothermal heat pump water / air and a traditional heat pump air / air. After dimensioning the vertical geothermal probes with the major theoretic systems (German regulations VDI 4640, IGSHPA and ASHRAE), two different models have been developed: one for the heat exchange between the heat transfer fluid and the subsoil, and another one for the calculation of the energetic, economic and environmental performances of both the systems. Under examination is a commercial building virtually placed in three different Italian climatic regions (in the northern, central and southern area of Italy).