It is estimated that about 140 millions of heat pumps are working in the world. This evidences that the equipment conceived by Lord Kelvin back in 1852 has finally penetrated into the market. In the last years the heat pump was improved both regarding the thermal exchange surfaces, the compressor and control and defrosting systems. Thus not only the COP was strongly improved but also seasonal performances. Further equipment improvements are aimed to better exploit the properties of new refrigerants, utilising even the pressure drop between condenser and evaporator, usually dissipated by throttling. Gas driven heat pumps were also improved with higher efficiency i.c. motors and much longer maintenance intervals. Also absorption heat pumps are now available in many different models suitable to different applications. Probably the most of improvements took place with lower heating temperatures and the use of cold sources more suitable than the outside air, above all surface and underground water, the ground and the recovery in mechanical ventilation systems.