Heating and air conditioning are among the largest energy uses in modern Buildings. A realistic view of the efficiency potential for this equipment is essential far rational estimates of the viable energy reductions in commercial and residential buildings. Although Lord Kelvin identified the heat pumping process almost 150 years ago, practical hardware did not come into wide-spread use until after World War II. Today, several million heat pumps are produced each year on a worldwide basis. The discussion of heat pumps will include the nonreversible heat pump that could, in theory, be cooling-only or heating-only. If this wider view Is taken, then there are over 30 million units produced each year. The factors to be considered in the forecast of technology are: the function of a heat pump, the diversity of demand, curb-side energy source far power, energy source / sink for the thermal process, basic heat pumping process, compression mechanism, thermal distribution subsystem, efficiency trajectory, frosting and refrigerant selection. In each of these areas, the scope will be global, since the governing physical laws do not change as one crosses national borders.