This paper will overview the system level testing of the Passive Thermal Control System (PTCS) for the Space Station Freedom (SSF) and highlight the design concepts that have evolved from the testing. The specifics of several system level tests, simulations and prototype will be discussed in detail.A series of development tests have been conducted under simulated space environments and operating conditions to verity the PTCS design, which is an essential component in ensuring the survivability of the Space Station Freedom in the hostile space environment for a mission life of 30 years. The Passive Thermal Control System is required to maintain the interior surface temperatures above 60°F (15.6°C) to prevent condensation and below 113°F (45°C) to protect the crew. In addition, the PTCS design must satisfy a thermal temperature range requirement of −250°F to +300°F (−156.7°C to 148.9°C) and a acceptable thermal leak requirement from the pressurized modules.The Passive Thermal Control System consists of multilayer insulation (MLI), coatings, thermal isolators, thermal insulation and heaters used to maintain structural temperatures in the acceptable ranges based on the local thermal environment. The MLI, which is a insulation blanket applied to the exterior of the pressurized elements to thermally isolate the internal manned environment from the space environment, was tested for venting, grounding, thermal performance, damage tolerance/leak detection and manufacturability. Thermal control coatings applied to the element meteoroid/debris (M/D) shields to locally bias the shield temperature to acceptable ranges, were tested for thermal performance, process verification and feasibility. The thermal insulation used on internal tubing to prevent localized condensation was tested for thermal performance and feasibility. Finally, the heaters used to prevent condensation on windows and internal surfaces at different module location in addition to providing freeze protection in various fluid line, were tested for thermal performance and integrated operations.