There is a renewed interest, expressed by ISECG, to return to the Moon and establish a longer human presence on the lunar surface. Long term missions require supplies, which can be transported to the Moon by a new promising solution: the Solar Electric Propulsion (SEP) stage. The goal of this study is to create a conceptual system design (phase 0) of such a SEP stage for Earth-Moon cargo transfer missions. The design is driven by the propulsion and electric power system, which are the main focus of the study, although all other subsystems are also covered to obtain a holistic system design. Designing a SEP stage is a multi-disciplinary task, in which the trajectory analysis, propulsion system and electric power system are tightly coupled. A mission analysis program is created for simulation of the spiral transfer, while flexible design tools are developed to create the conceptual design. The tools provide the possibility to quickly evaluate a different mission scenario, such that the most suitable scenario, in consultation with the customer, can be selected. Aconceptual design is created that meets the mission objectives and requirements. During the design phase it was identified that the concept of a solar electric propulsion stage, accommodates some critical issues and technological challenges. Especially the high power demand, leads to the usage of highly conceptu l power conditioning techniques, which still have to be proven in space, possibly by a precursor mission. It was also found that the SEP stage, compared to a chemical rocket, is capable of transporting 32% more payload, while having the same initial mass. This number can even increase to 90% in case a different mission scenario, with a longer transfer duration, is selected.