The Earth Observing-1 (EO-1) spacecraft is the first earth orbiting spacecraft in NASA's New Millennium Program. The New Millennium Program is part of the agency's Mission to Planet Earth enterprise, a series of space missions designed to enhance our knowledge of the Earth and its environmental systems. The EO-1's mission is to employ advanced remote-sensing technologies, including hyperspectral and multispectral imaging techniques, to develop highly accurate terrestrial images. In order to accomplish this mission, the spacecraft contains three primary instruments: Advanced Land Imager (ALI), Atmospheric Corrector, and Hyperion. The bus supporting these sensors is part of a 3-axis stabilized, nadir pointing spacecraft that employs an articulating solar array to provide a constant voltage, regulated power bus. EO-1 also contains several new technologies such as a carbon-carbon radiator and a pulsed plasma thruster that will be verified as part of the secondary mission objectives.Following the scheduled launch in July 2000, EO-1 will fly in a sun-synchronous, 705 km circular orbit, one minute behind the LANDSAT-7 Spacecraft. Using data from LANDAT-7, researchers can validate the ALI technology and evaluate the Hyperion performance over EO-1's one-year primary mission duration.In order to provide a comprehensive verification of the EO-1 spacecraft thermal design and system operational functionality while meeting the stringent project cost and schedule constraints, a creative and cost-effective testing approach was taken. EO-1 spacecraft thermal vacuum testing took place at the NASA Goddard Space Flight Center between October 1, 1999, and October 19, 1999. The actual test profile consisted of one hot and two cold balance cases and three hot and cold system functional and performance test plateaus. Simulation of the space environment was accomplished in a vacuum chamber using a combination of IR heater and cryopanel shrouds, body-mounted test flux heaters, and the liquid nitrogen flooded chamber cold wall. Electronics and instruments were operated at power levels expected during the various flight mission scenarios for both functional and balance testing.The overall test results were excellent indicating that all spacecraft flight hardware performed nominally with all thermal requirements met. Pre-test thermal model predictions compared favorably with thermal balance simulation data, therefore post-test thermal model correlation efforts were minimal. The test results showed a cold biased design; however, adequate heater sizing demonstrated during cold operational and safehold conditions dictated only minor radiator area reductions for system power optimization.This paper describes the overall test philosophy, thermal test objectives, configuration, results, TMM correlation, and also the relevant conclusions and corresponding lessons learned that were derived from both the thermal balance and functional testing. In addition, integration and test practices implemented to expedite schedule are discussed along with their corresponding time savings.