The Cassini spacecraft, NASA's mission to investigate the Saturn system, has undergone a system-level thermal balance test program to permit verification of the engineering subsystem thermal designs in the simulated worst-case environments. Additionally, other objectives such as functional checkouts, collection of thermal data for analytical model adjustment, vacuum drying of propellant tanks, and flight temperature transducer verification were also completed. In the interest of cost and schedule, transient off-Sunpoint conditions were not tested.The testing demonstrated that the required system resources such as heater power and radiator area were adequate for all engineering subsystems. The only changes required from the results were related to the operation of some of the subsystems. In the instance of the thruster cluster assemblies, allowable flight temperature limits were exceeded for the assumed operational environment. The cause was attributed to a non-flight use of all the catalyst bed heaters. In order to assure that the propellant control assembly would be controlled by a computer-controlled heater within a specified temperature range, a change in the controlling flight temperature sensor set-point was instituted. Additionally, suitable heater capability was demonstrated for the propulsion tank and main engine thermal conditioning.