The European ARIANE 5 launcher is designed to put a payload mass of about 6.0 t into geostationary transfer orbit. It consists of a lower central stage with two solid propellant strap-on boosters and a single-engined storable propellant upper stage. The ARIANE 5 was successfully launched on October 30th, 1997.This paper deals with the thermal protection of the ARIANE 5 upper stage. The tanks, the actuators, the instrumentation and electronic equipment have to be protected from the heat input of the hot engine nozzle during the 1100 second thrust phase and maintained operational for a further time period for passivation or re-ignition of the engine.The paper provides a short history of the upper stage development and the thermal design for the configuration of the qualification flights.For the production phase the optimization potential with regard to mass saving and simplified integration has been investigated. Thermal design alternatives are revealed and it is shown where the insulation hardware could be further improved.