The current status of the research on mercury cadmium telluride (MCT) defects is reviewed by summarizing and analyzing the related papers published in recent years. MCT offers significant advantages over other similar semiconductors. This has made it to be the most widely used narrow gap material in infrared focal plane array (FPA) devices. The defects in MCT epilayers can inhibit the pixel performance and operatability of the FPA fabricated with this material. The type, orientation, and growth temperature of the substrate etc. are the factors which have apparent effect on MCT epilayer quality. To grow high quality MCT epilayers, different factors which may result in defects should be further understood and controlled properly. Significant progresses have been made in the epitaxial growth of MCT with the aid of various fabrication and characterization techniques.