Heat pipes and thermosyphons can be very effective heat transport devices for transferring the thermal energy of the Next Generation Nuclear Plant to a hydrogen production plant and/or other process heat applications. These devices operate nearly isothermally, transporting large amounts of thermal energy with little or no temperature drop. A dimensional analysis of the thermosyphon and the heat pipe is presented in this paper. Dimensional analysis is a valuable mathematical technique useful in research work for design and conducting model tests. This analysis yielded two terms - Er and EM - particular to the operation of these devices in addition to those commonly used in many heat transfer applications. The Er term relates the latent heat of vaporization to the pressure drop across the device, while the EM term relates the latent heat of vaporization to the capillary pressure. The significance of these two terms is discussed. The universal nature of these numbers should be useful in increasing the fundamental understanding of both the thermosyphon and the heat pipe.