An overview of the external Active Thermal Control System (ATCS) for the International Space Station Alpha (ISSA) is presented. Changes to ATCS architecture made during the transition from the Space Station Freedom (SSF) program have significantly affected system design and operational characteristics. ATCS architecture, described in a system schematic, shows the results of this evolution based on new requirements implemented for the ISSA program. Two independent, single phase ammonia thermal loops replace the three independent, two phase ammonia loops used for SSF. The concept of packaging system components in orbital replaceable units (ORUs) is maintained, however an additional module replaces the interface with the environmental control system previously used as the source for pressurant to the system. The functions and characteristics of the ORUs are summarized. Avionics, fluids, and structural interfaces have been simplified to minimize crew extra-vehicular activity time during system startup and maintenance. Concepts for providing on-orbit maintenance of ATCS hardware is discussed. The diverse test program discussed in the paper is being implemented to verify ATCS operation at component, assembly, and system levels.