This analysis of invention is the result of a study of patents, patent applications and some other technical and scientific lite rature published in the United States, Japan, Great Britain, Western Germany and France, as well as European (Munich) and PCT (Patent Cooperation Treaty) patent applications, in the period from about January 1975 to about January 1985. It provides a systematic review of research activities on the structural and operational development of heat pipes in the period covered, and on their applications. The study is in two parts: Part I relating to aspects of indi vidual heat pipes and Part II concerning the application of heat pipes to heat exchangers and mechanical and electrical components, in which the construction and operation of the in dividual heat pipes are of secondary importance. The material studied during the search has been derived from the search files of the European Patent Office, Rijswijk, The Nether lands, classified in accordance with the following international patent classes (IPC): F 28 D 15/02 relating to heat exchange apparatus with the intermediate heat transfer medium in closed tubes passing into or through the conduit walls in which the medium condenses and evaporates, e.g. heat pipes. F 24 J 2/32 relating to heat pipes of solar heat collectors. Vlll G 21 D 5/02 relating to arrangements of nuclear reactor and engine, in which reactor produced heat is converted into mechanical energy and in which reactor and engine are structurally combined."