This book is the direct result of a NATO Advanced Study Institute on "Heat Pumps" held at Les Arcs, France, June 1975, under the director ship of Profes sor E. Camatini, Politecnico di Milano, Italy. Leading world experts in the field were invited to attend this Institute and to lecture on the heat pump and its potential applications in the broader context of energy conservation. The lectures focussed on today's energy problems, fundamental principles of thermodynamics and their application to the heat pump, potential heat sourc es for heat pumps, alternate heat pump drives, heat transport and special applications in some areas where considerable savings could be obtained. When the Institute was planned, publication of the lectures in the form of a book was not envisaged. It was felt, however, during a round-table discussion with the lecturers that there was a definite need for publication of the lectures presented, since a comprehensive scientific Itechnical book on the heat pump seemingly did not exist. The resulting volume might not provide the reader with a full picture of the heat pump in the sense that the technical design of individual components and existing large installations of heat pumps are not addres sed in detail.