Devices that respond to incident ultraviolet, visible, or infrared electromagnetic radiation by giving rise to an output signal, usually electrical. Based upon the manner of their interaction with radiation, they fall into three categories. Photon detectors are those in which incident photons change the number of free carriers (electrons or holes) in a semiconductor (internal photoeffect) or cause the emission of free electrons from the surface of a metal or semiconductor (external photoeffect, photoemission). Thermal detectors respond to the temperature rise of the detecting material due to the absorption of radiation, by changing some property of the material such as its electrical resistance. Detectors based upon wave-interaction effects exploit the wavelike nature of electromagnetic radiation, for example by mixing the electric-field vectors of two coherent sources of radiation to generate sum and difference optical frequencies. This discussion concentrates on photon and thermal detectors.