In virtually every city the number of existing buildings dwarfs the number of new buildings being built every year. This huge stock of existing buildings and the building sites on which they sit, most of which were not designed for resource efficiency, offer many opportunities to make sustainability improvements. Even buildings that are relatively new frequently present a number of economically beneficial opportunities for efficiency upgrades, which help meet greenhouse reduction goals and also pay for themselves in reduced operating costs. Moreover, buildings with high energy efficiency and whole-building optimization improve occupant quality of life, employee productivity, and have a higher resale value.