An analytical and experimental study of the single component, single phase closed thermosyphon with laminar and turbulent boundary layer flow is presented. In the experimental study, a transparent test cell was employed so as to directly observe the transport processes which were found to be basically convective. Experiments, with Prandtl numbers from 4 to 940, were conducted for overall temperature differences of one, three and nine degrees centigrade for which the temperature field was measured. A convective model, showing good agreement with experimental heat transfer data, is formulated for the analytical study.