Electrochromic devices (switchable mirrors) that exhibit large dynamic ranges for reflectance in the visible and infrared regimes can now be made using a variety of materials. Devices incorporating these films can be used to improve energy efficiency in buildings and vehicles by controlling the flow of heat not only through windows and skylights, but also through opaque roof and wall panels. Switchable mirrors based on three completely different chemical reactions have been developed at LBNL. The best known example utilizes conversion of a thin metal film to an insulating and transparent metal hydride by either direct exposure to hydrogen gas ("gasochromic") or by applying a potential to the metal film in contact with an appropriate electrolyte ("electrochromic"). Others involve electrochromic lithium compounds and copper oxides. Tom Richardson, Jonathan Slack, Jan Isidorsson, and Oliver Meier of the Building Technologies Department's Windows and Daylighting Group are in the second year of an intensive effort to bring the electrochromic metal hydride switchable mirror to the marketplace. The fundamental principles of operation, materials issues now being addressed, and prospects for success will be discussed.