This chapter discusses the technological application of a class of materials with a chameleon-like nature, that is, they exhibit the properties of metals under certain conditions of temperature and pressure, and semiconductor-to-dielectric properties under other conditions. Many materials exhibit this behavior, most notably the transition metal oxides and sulfides. Typically, the transition from one state to another in transition metal oxides is accompanied by a sharp change in electrical conductivity (as large as 107 in some oxides of vanadium), as well as changes in other physical properties such as crystalline symmetry. The changes in electrical conductivity alter, in turn, IR transmittance, and some of these effects extend into the visible spectrum. A material such as this, whose transition occurs at the appropriate temperature, would be useful for solar energy control in buildings. For example, a coating of thermochromic (TC) material on glass would transmit solar energy at temperatures below its transition temperature (Tt), and when the temperature rises above Tt, the TC material would reflect the incident solar energy. Thus, solar influx would be high at low ambient temperature and low at high temperature. Though very few of these materials have Tt in the range required for such an application, we can adjust Tt by using dopants.