Advance thermal management systems are being developed to optimise the energy balance within aircraft. This is being done in parallel to the concept of the More Electrical Aircraft (MEA) which has been developing throughout the last decades.The objective of such complex systems is to use efficiently the hot and cold sources available within the air vehicle to reduce the engine fuel consumption. A reduction of electrical power consumption, minimisation of weight, optimisation of aircraft aerodynamics (for example RAM inlets area minimisation) and the reduction of bleed air from engine all result in a fuel consumption saving.Any thermal management system to optimise energy consumption implies complex and advanced systems. This requires a high engineering effort to design and integrate the system within an aircraft due to the large quantity of variables and interfaces that need to be taken into account.Models and simulations are essential from the beginning of the system development and design phase. However, such models are also useful along the entire system life cycle (i.e. testing, verification, in service, system modifications). They must be evolved and improved to adapt to each phase and the results need to be checked with laboratory and prototype data.In addition, the use of dynamic models over classic steady models permits designers to take into account the thermal inertias in order to dimension the systems with a more realistic behaviour in accordance to the intended operation. It also allows considering the different flight mission profiles that the air vehicle shall cope with and includes as variables the heat loads that need to be dissipated. This leads to an optimised system for the real scenario and not an oversized one against the worst case scenario.The present paper describes the use of a dynamic model to concept, dimension and design a thermal management system.