For various infrared photoelectric detectors operating in the wavelength region from 1 μm to 30 μm, mercury cadmium telluride (MCT) has become the most important semiconductor material. To obtain the best performance, the dark current in the MCT detectors must be minimized. By summarizing and analyzing the related references published in recent years, the progress of the research on MCT dark current is presented and the understanding and comprehension of the dark current in MCT photodiodes are discussed.