This book contains the texts of the lectures which were givenat the Nato Advanced Study Institute on Advanced Heat Pumns which was heldat Espinho, Portugal in September 1980. Aprevious NATO Advanced Study Institute on the topic of heat pumps had been held in 1975. The significance of heat pumps with respect to energy conservation was the main topic of this Institute. In 1980it was felt that considerable research had to be done in order to be able to produce more energy efficient, less costly and more widely applicable heat ~umos. This requires a good understanding of the functioning of the types of heat pumps available. The simultaneous coverage of the basic fundamentals of heat pumps of different drive in one lecture series therefore was the goal of the 1980 Advanced Study Institute. Only a few lectures were devoted to heat pump applications. The lectures on heat pump applications were intended to ~ive only a short overview. They were supplemented by lectures on the latest developments on vapour compression as well as sorption systems.