A heat pipe is a heat transfer device characterised by very high effective thermal conductivity up to 500 times greater than that of a solid copper bar of the same length and outer diameter. The focus of this paper will be on phase-change from liquid to vapour in the evaporator sections of heat pipes. The boiling phenomena in heat pipes and thermosiphons are analysed in relation to the general boiling correlation presented by Rohsenow  and experimentally observed by Nukiyama . Different specific correlations developed for boiling in thermosiphons are compared with the general theory. The transition from nucleate boiling to film boiling has been experimentally observed and analysed by the authors. Future work is needed to develop a general theory of boiling in heat pipes and thermosiphons.