ESDU 80013 is one of a series concerned with heat pipes. It reviews the various types of heat pipes, outlines their mode of operation, and indicates operating limits. It also indicates possible applications. It discusses types of wicks and appropriate working fluids, including the use of the Figure of Merit to assess them, and the importance of compatibility with materials. Transient behaviour is considered, and resistance to vibration. Other data in the series give methods for calculating the performance of capillary-driven designs, two-phase closed thermosyphons, data on wick properties and data on the physical properties of working fluids.