This chapter provides an overview of the diseases of crustaceans caused by fungi and oomycetes. Knowledge of fungal diseases of crustaceans lags behind that for viruses and bacteria. Various diseases caused by fungi have been reported including black gill and burn spot diseases caused by Fusarium spp., and lethargic crab disease caused by Exophiala cancerae and Fonsecaea brasiliensis. Other diseases caused by fungi include mycosis of shore crabs (Carcinus maenas) and a similar disease of edible crabs (Cancer pagurus) caused by an Ophiocordyceps-like fungus. Over 60 species of microsporidians infect crustaceans including Ameson spp., Enterospora spp. and Pleistophora spp. Finally, a key disease of crayfish is crayfish plague (krebspest) caused by the oomycete, Aphanomyces astaci, that has adversely affected native populations of crayfish, particularly in Europe. Other oomycete pathogens include Lagenidium, Haliphthorus spp., Halocrusticida spp., and Halioticida sp. that affect eggs and larvae of aquatic crustaceans.