Infrared detectors are discussed in terms of an equivalent circuit consisting of a photocurrent generator in series with a gain mechanism, both shunted by the detector internal impedance. Detectivity, the usual measure of infrared detector sensitivity, is defined in terms of the parameters of this equivalent circuit. The properties and performance of the most common and best developed infrared sensitive photoconductors and photodiodes are then described in terms of these parameters. Both normal and low background performance are considered, with emphasis on maximum sensitivity in the low-to-moderate frequency region. Short discussions of the current status and recent developments in thermal detectors, high-speed detectors, and infrared detector arrays are included. The importance of preamplification is stressed throughout, and preamplifier principles and current practice for the various types of detector are separately discussed.