Publisher Summary For many years now, there has been interest in the use of thermal radiation for military purposes, both for the detection and location of hot or warm objects and the use of the emission from objects near room temperature for thermal mapping (thermography). Interests in the infrared are not confined to military applications. In addition to an interest in solid-state spectroscopy, the application of far infrared techniques to plasma diagnostics have also been studied in controlled thermonuclear research. One common requirement that these applications have shared has been the need to develop infrared detectors with suitable characteristics. It is convenient to classify infrared detectors as thermal detectors or as photoconductive ones. In thermal detectors, the absorbed radiation raises the temperature of the detector, thereby changing the temperature-dependent property of it. In photoconductive detectors, the absorbed radiation induces electronic transitions that lead directly to a change in electrical conductivity. This chapter covers the development of the photoconductive detector, focusing on some of the more recent developments of thermal detectors. When choosing a detector for a particular application, the main factors to be considered are spectral response, sensitivity, speed of response, and convenience in use. Photoconductive detectors for wavelengths shorter than and longer than 10 pm, respectively, are highlighted.