This chapter focuses on how a building designer can incorporate concern for the operating energy requirement of a building at the design stage. The mechanical and electrical systems of a building consume energy to provide transportation, lighting, ventilation, heating, and cooling for the building occupants. These systems have an operating energy requirement that is, over the life of the building many times larger than that needed to construct the building, the capital energy requirement. In nonresidential construction the capital and operating energy costs of energy-consuming systems often comprise a significant fraction of the total building cost. In a climate of high energy costs, the operating energy consumption is a vital factor in choosing the "best" building design. Determining the capital cost of conventional energy-consuming equipment is possible within the constraints of inflation and normal business uncertainty. Predicting the operating cost is a much more difficult problem as both energy use and future energy prices are unknown.