The weakest part of the building’s energy system is normally its windows and glass facades, which let in too much energy so that cooling is needed or let out too much energy so that heating is needed. The windows and glass facades are required to create indoors-outdoors contact and for day-lighting, so the energy issue cannot be solved by making the windows small. This talk outlines recent research and development to create windows that can adjust over the day and season, so that they let it an appropriate amount of visible light and solar energy in order to maximize the energy efficiency jointly with providing good indoor comfort. Two technologies will be introduced—electrochromics and thermochromics—which make it possible to vary the throughput of light and energy by use of a low electrical voltage or as a response to a change in temperature, respectively. Device designs, materials, and progress towards low-cost manufacturing are discussed.