This paper will review the current state of the art of internal photoemission infrared detectors. That is, detectors which sense in a multi-step process, beginning with photon absorption in an electrode and terminating in emission of an excited carrier over a potential barrier into a semiconductor depletion region. Internal photoemission (PE) is a surface barrier sensing process similar to vacuum photoemission. This differs from competing bulk detection processes where both photoabsorption and carrier measurement takes place in the semiconductor. The PtSi/p-Si Schottky photodiode is the most advanced and best documented infrared PE device. Our interest is more general however and we will include devices where the photoemission 'electrode' may be a metal, a metal silicide or a degenerate semiconductor.