Publisher Summary The success in semiconductor technology has rested on the application of single crystals of the appropriate material. The startling revolution of fiber-optic communications would have been stillborn without the availability of long-wavelength lasers based on single crystal material in the indium gallium arsenide phosphide materials system. This chapter examines the importance of single crystal mercury cadmium telluride in the development of effective thermal imaging systems. The twentieth century was dominated by the cathode ray tube, while the twenty-first century has so far been dominated by the flat panel display. The nature of infrared detector materials presents examples of the lead chalcogenides, which seem to work best in the form of polycrystalline thin films deposited on glass substrates and the physics of their electrical conduction behavior. Amorphous and polycrystalline semiconductors play an essential role. A particularly important development has recently appeared in the literature, in the shape of a large-scale thin film solar power plant installed in Nevada by the California-based company Sempra Generation. A similar technology could surely be used on every house and factory roof.