Shell fishes like crustaceans and molluscs are often prone to shell diseases. Among crustaceans, crabs, lobsters and shrimps are largely infected by infectious pathogens and various environmental stresses. Viruses are commonly seen in shells of shrimps, whereas dinoflagellate is predominant in lobsters. Crabs are infected in higher extend than any other crustacean. Shell disease syndrome in brachyuran crabs is characterized by damaged external manifestation of colored lesions in the exoskeletons. Bacteria (Vibrio, Pseudomonas and Aeromonas), viruses (WSSV), fungi (yeast) and several other pathogens influence the higher percent of shell disease in crabs. Crabs with shell diseases are prone to internal damages causing variation in haemocyte counts and histopathological alteration in internal tissue and organs. Infections in crab may lead to great economic loss. This review discusses about the causes, types, histopathology, genetic variability, hematology, humoral defense mechanism and host susceptibility hypothesis of brachyuran crab shell diseases.