Hemogregarines of the genus Hepatozoon are intraerythrocytic apicomplexan parasites that have been described from all groups of tetrapod vertebrates. Gametogenesis, fertilization, and sporogonic development, which culminates in the formation of polysporocystic oocysts, occur in the gut or hemocoel of a hematophagous arthropod definitive host. Merogonic development occurs in the internal organs of vertebrate hosts after they ingest these infected arthropods. The presence of cystic stages, observed for many Hepatozoon species, increases life cycle complexity and exploits the feeding behavior of vertebrate hosts. The inconsistency of morphological characteristics of these parasites, especially those associated with gamont structure, coupled with low host specificity of the parasites for their invertebrate and vertebrate hosts, have rendered species differentiation difficult. A systematic review of the hemogregarine complex has resulted in the expansion of the genus Hepatozoon to include all members of the genus Haemogregarina that infect amphibians, snakes, lizards, crocodilians, birds, and mammals.