This paper describes how the hybridization of conventional vehicles affects the cooling system layout. Combustion engines, electric drives, power electronics and batteries run at different temperature levels, thus, the heat loss should dissipate to individual cooling circuits. To ensure safe and reliable operation a thorough design of the cooling system is required. In the second part of the paper, the system layout for a SUV is shown. The hybrid vehicle presented here uses the power from an internal combustion engine and two electric drives. It features three different cooling circuits. The layout process is supported by a sophisticated simulation technique: by means of co-simulation of the mechanical, electrical and thermal systems, the system behaviour has been analyzed under various ambient conditions and vehicle loads. The results of these simulations will be the basis for the final system layout and thermal management strategy of the vehicle.