The thesis presents the development of system-level simulation models for the preliminary design of the Thermal Management System (TMS) of a hybrid-electric aircraft. A state-of-the-art review is first conducted, identifying system objectives and available technologies for managing the thermal loads of electrified propulsion components. The modelling work is carried out within the framework of HERA, a European-funded project targeting a regional hybrid-electric aircraft, which defines two configurations: the Ultra Efficient Regional Aircraft (UERA), based on a hybrid-electric propulsion architecture, and the Ultra Clean Business (UCB) aircraft, based on a hydrogen-fueled architecture incorporating fuel cells. The thesis specifically addresses the Battery and Power Electronics TMS for the UERA and the Fuel Cell TMS for the UCB.