The Thermal Test Facility (TTF) at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, was designed and constructed using a whole-building energy design approach. This approach treats a building as a single unit, not as a shell containing many separate systems. It relies on the use of energy simulation tools for optimization throughout the design process, and requires the involvement and commitment of the architect, engineer, and owner. It can produce a building that requires substantially less energy than a building designed and constructed with conventional means. TTF operating costs are 63% less than those of a code-compliant basecase building. These savings were achieved by implementing an approach that optimized passive solar technologies and integrated energy-efficient building systems. Passive solar technologies include daylighting, high-efficiency lighting systems, engineered overhangs, direct solar gains for heating, thermal mass building materials, managed glazing, and a good thermal envelope. The energy-efficient heating, ventilating, and air-conditioning (HVAC) system, designed to work with the building's passive solar technologies, includes ventilation air preheat, ceiling fans, indirect/direct evaporative cooling, and an automatic control system. This paper focuses on the design features of the TTF and the results of tests conducted on the TTF since its completion in 1996. …