Efforts in infrared (IR) detector research are directed towards improving the performance of single-element devices and large electronically scanned arrays, and to increasing their operating temperature. Another important aim is to make IR detectors cheaper and more convenient to use. Among the many different types of materials used in IR detector technology, narrow-gap semiconductor materials have a privileged position. In this monograph, investigations of the performance of narrow-gap semiconductor photodiodes are presented, and recent progress in different IR photodiodes technologies is discussed: HgCdTe photodiodes, InSb photodiodes, alternatives to HgCdTe III-V and II-IV ternary alloy photodiodes, lead chalcogenide photodiodes, and a new class of photodiodes based on two-dimensional solids. Investigations of the performance of photodiodes operated in different spectral regions are presented, including short wavelength IR (SWIR), 1-3mm; medium wavelength IR (MWIR), 3-5mm; long wavelength IR (LWIR), 8-14mm; and very LWIR (VLWIR), above 14mm.