The Space Shuttle Orbiter Active Thermal Control Subsystem (ATCS) is designed to provide vehicle and payload cooling during all phases of a mission and during ground turnaround operations. The system consists of dual Freon coolant loops (FCLs) that transport heat from the various heat sources to four major heat rejection devices. The waste heat from these sources is transferred to the FCLs by coldplates and heat exchangers and is then rejected by one of the four heat sinks, depending on mission phase. During the first 39 flights of the Shuttle program, the ATCS has demonstrated excellent operational performance, although several flight anomalies and maintenance problems have been experienced.This paper describes the operation of the Shuttle ATCS and some of the problems encountered, their causes and resolutions. Principal emphasis is given to the major problems that have been encountered with the degradation of Freon flow rate on the Orbiter Columbia; the Flash Evaporator Subsystem (FES) mission anomalies which occurred on STS-26 and STS-34, and operational experience; and problems encountered with the Ammonia Boiler Subsystem (ABS).