The thesis focuses on the preliminary design of the Thermal Management System (TMS) for a hybrid-electric aircraft. Initially the state-of-art review is widely investigated, identifying the objective of the system as well as technologies currently available. Thus, it is demonstrated that Meredith Effect and Vapor Cycle Machine are key elements for next generation aircraft. Subsequently, case study is shown, in order to drive the TMS design towards right aims. The case study is the European funded project named HERA, which aims to develop a 80 passenger hybrid-electric aircraft. Following this chapter a preliminary analysis is led, involving analytical analysis of technologies previously identified as those with the most potential.