Today's automotive cooling systems are designed for the highest necessary heat rejection. This operating state is reached only during 3-5% of the operating time. Thus in most cases the thermal situation is subcritical and the engine is overcooled. Under these operating conditions a demand-responsive engine cooling system promises a high potential for saving energy, increased passenger comfort and environmental compatibility. To realize this potential new components and control strategies are needed. The present paper shows the potential of a demand-responsive cooling system. The requirements of the system and its components will be specified. The realization of this new cooling concept and experimental results regarding reduction of fuel consumption and also on improved driving comfort will be presented.