Window system design and operation have a major effect on energysuse in buildings as well as on occupants thermal and visual comfort. Window performance will be a function of optical and thermalsproperties, window management strategies, climate and orientation,sand building type and occupancy. In residences, heat lossscontrol is a primary concern, followed by sun control in moressoutherly climates. In commercial buildings, the daylight providedsby windows may be the major energy benefit, but solar gainsmust be controlled so that increased cooling loads do not exceedsdaylighting savings. Reductions in peak electrical demand and HVACssystem size may also be possible in well-designed daylightedsbuildings. Improved analysis tools and more extensive experimentalsdata on window performance in buildings are necessary tosimprove design decisions and guide new product development.