Coccidia are intracellular parasites belonging to the suborder Eimeriorina. In fishes these parasites are represented by seven genera: Calyptospora, Goussia, Eimeria, Epieimeria, Cryptosporidium, Crystallospora, and Octosporella. Members of these genera include species that have both direct and indirect life cycles, those that occur in both intestinal and extraintestinal sites, and some that sporulate endogenously as well as exogenously. Some of these piscine coccidians are also capable of producing severe histopatholgical changes in infected hosts and pose a potential threat of coccidiosis in fishes reared under intensive conditions. Examples include Goussia carpelli infections in cultured carp and gold fish and Calyptospora funduli infections in cultured Gulf killifish. Current taxonomic classifications, life cycles, associated histopathology, and other biological parameters will be discussed.