The Thermal design of the ENVISAT-1 ASAR Active Antenna has provided many engineering challenges. The selection of the Thermal Control has been complicated by the need to dissipate the high power (1300 watts) generated by the active equipments that are mounted on the Antenna Tile Sub-System directly behind the radiating surface. This high power has to be dissipated from the Antenna whilst minimising heater power consumption. The final design uses passive Thermal Control for dissipating the heat from the equipments during operating modes, via radiation from the Active face of the Antenna towards the Earth. This has required evaluations of possible Thermal finishes and resulted in a black painted Earth facing Active radiating face. Heat loss by conduction to the ENVISAT-1 platform has also been minimised in order to ensure that ASAR has negligible impact Thermally on the platform. Further decoupling between Panels and between Tiles has allowed the ASAR Antenna Thermal design to be verified at SubSystem level rather than at full Antenna level. The Thermal Control of the Antenna in OFF modes, using heaters and thermostats, had to be optimised in order to minimise the number of heater circuits and the heater power consumed. The Thermal Control must also allow for the Antenna to be launched stowed and then be deployed once on orbit, hence Thermal Control of the release and deployment mechanisms, up to completion of the deployment, must also be considered. Protection for the Antenna is achieved using Multi-Layer-Insulation (MLI) in which the number of layers and material finishes have been chosen after detailed trade-off studies.