The uniqueness of the International Space Station (ISS) lies on the fact that the spacecraft is built incrementally on-orbit through a series of assembly missions. During the construction phase, many systems change configuration as the modules are added to the ISS.The function of the Thermal Control System (TCS) is to maintain thermal conditions within design parameters for the ISS and user payloads. Within the TCS, the External Active Thermal Control System (EATCS) collects the heat from the Interface Heat eXchangers (IHX) and system equipment coldplates, transports the heat to the EATCS radiators and rejects the heat to space. The EATCS is comprised of two single-phase anhydrous ammonia loops. Each loop includes, among other items, several liquid -to-liquid interface heat exchangers. This paper will present the dynamic characteristics of the EATCS and the challenges to its stable dynamic control as the ISS reaches the assembly complete configuration.