As the CubeSats have been getting more energy dense over the years, a greater need has come for thermal control on CubeSats. Phase Change Materials (PCMs) are researched in this thesis as an energy dense passive Thermal Storage Device (TSD). This energy density comes from the melting of the PCM. To determine the feasibility of this TSD. The thermal storage device needed to be able to keep certain components from overheating while keeping other components from getting too cold. This thesis successfully tested that the PCM was able to store heat from a radio and use that heat to keep a CubeSat battery warm. The TSD is meant to stabilize the temperature of all components connected to it by maintaining a temperature around the melting temperature of the PCM. Prototypes were made using eicosane wax, it is a paraffin wax with a melting temperature of 36.7∘C. This PCM was chosen due to its safe and predictable properties, and because of its previous use in the space industry. Two different casings where tested with two different filler materials and casing sealing methods. Although the filler materials had a relatively small effect on the thermal response of the casing at the heat fluxes tested. The fin filler material used less mass and was simpler to manufacture than the honeycomb filler. There was a large difference in the sealing method of the casing that was glued together sealed better than the one laser welded together, the casing which had fill ports that were clamped with a thread with a rubber filament O-ring between them sealed better than the one that was glued on. Although the degree to which this could be tested was limited and is therefore difficult to draw definitive conclusions over. The tests proved that the TSD was much more effective than traditional heat sinks at storing heat. This will allow energy dense CubeSats with large intermittent heat-loads to be designed for their average temperatures and not for extreme peaks and valleys. The complexity this casing adds to the CubeSat is more than traditional heat sinks but much less complex than active thermal control systems. The casing thermal response could be predicted within a 5∘C accuracy using a simple Finite Element Method (FEM) model using the built-in conduction and Phase Change Material modules in Comsol. The remaining obstacle with this research project is that more work needs to be done to be able to prove that the casing is completely hermetically sealed and will not contaminate other hardware while in operational.