Heat-pump operation in Great Britain is reviewed from the point of view of the electricity supply industry. The factors which have to be taken into consideration for possible applications are discussed, e.g. sources of low-grade heat, capacity required and special methods of utilization.British experience, including the results of laboratory investigations on heat transfer from the soil and other aspects, is described. Details of construction and operation of a practical 10 h.p. heat pump, installed to warm a laboratory building, and using the earth as its source of low-grade heat, are given. Observations of performance include load factor and maintenance and attendance.The economic aspect is dependent on the present state of development, but future improvements together with comparisons with other methods of performing the same duties are considered. The paper concludes with a discussion of the value of the heat pump as an electrical load.