The scope of this report is: - to give a short description of the possible operation principles and applications of heat pipes - to derive the governing equations for a model of a conventional heat pipe (including the limits to the heat transfer capability) - to describe the theoretical derivation of the formulae for the various thermal resistances associated with heat transfer in and by heat pipes and the equations necessary for the calculation of the resulting temperature distributions - to present a design philosophy and illustrate the design procedures - to compile data that are required for a proper heat pipe design - to survey typical space applications of heat pipes (both existing systems and possible future applications) - to indicate specific topics and problems areas associated with heat pipe use in space - to make a listing of heat pipe manufacturers, suppliers and users In this report restriction will be made to the single fluid, cylindrical - so-called conventional - heat pipe. The extension to more (fluid) components heat pipes and variable conductance heat pipes is foreseen to be presented in a report later. Calculations performed for heat pipe - fin radiator - combinations will be reported separately. Recommendations for future work are given as a result of the information that has heen collated in the report.