Cleanrooms, critical to a wide range of industries, universities, and government facilities, are extremely energy intensive. Consequently, energy represents a significant operating cost for these facilities. Improving energy efficiency in cleanrooms will yield dramatic productivity improvement. But more importantly to the industries which rely on cleanrooms, base load reduction will also improve reliability. The number of cleanrooms in the US is growing and the cleanroom environmental systems' energy use is increasing due to increases in total square footage and trends toward more energy intensive, higher cleanliness applications. In California, many industries important to the State's economy utilize cleanrooms. In California these industries utilize over 150 cleanrooms with a total of 4.2 million sq. ft. (McIlvaine). Energy intensive high tech buildings offer an attractive incentive for large base load energy reduction. Opportunities for energy efficiency improvement exist in virtually all operating cleanrooms as well as in new designs. To understand the opportunities and their potential impact, Pacific Gas and Electric Company sponsored a project to benchmark energy use in cleanrooms in the electronics (high-tech) and biotechnology industries. Both of these industries are heavily dependent intensive cleanroom environments for research and manufacturing. In California these two industries account for approximately 3.6 million sq. ft. of cleanroom (McIlvaine, 1996) and 4349 GWh/yr. (Sartor et al. 1999). Little comparative energy information on cleanroom environmental systems was previously available. Benchmarking energy use allows direct comparisons leading to identification of best practices, efficiency innovations, and highlighting previously masked design or operational problems.