The HERMES Thermal Control System necessitates the adoption of both Active and Passive Thermal Control techniques.The ATCS (Active Thermal Control Section) utilizes fluid coolant loops collecting and transporting heat loads from the various sources to the available sink devices. The methods of analysis and trade-off applied in the preliminary phase of the design activities are described, with particular attention to the selection of the fluid, the architecture of the loop, the flow sequence of the equipment in line and the redundancy policy. The design driving criteria are then discussed, outlining the optimization efforts concerning the aspects of mass saving and safety.For the PTCS (Passive Thermal Control Section), the design philosophy together with analysis approach and techniques are described. Depending on the different constraints imposed by the various mission phases, the most appropriate insulation concept and required materials (foam/fibrous insulation and/or multi-layer insulation) are selected for each of the spaceplane compartments. The design is then optimized, with respect to mass and power consumption, using compartment level mathematical models, developed also to handle the particular problems posed by the spaceplane reentry.The resulting thermal design, both for ATCS and PTCS, is then presented and finally the development and testing activities discussed.