Theileriae are obligate intracellular protozoan parasites that infect both wild and domestic Bovidae throughout much of the world. They are transmitted by ixodid ticks, and have complex life cycles in both vertebrate and invertebrate hosts. There are a number of species of Theileria spp. that infect cattle; the two most pathogenic and economically important are T. parva and T. annulata, which cause East Coast fever/corridor disease and tropical theileriosis, respectively. Theileria parva affects cattle and water buffalo (Bubalus bubalis). African buffalo (Syncerus caffer) and cattle are important reservoir hosts for this organism. Theileria annulata affects cattle, yaks and water buffalo and is transmitted by ticks of the genus Hyalomma. Theileria orientalis/buffeli complex is now thought to consist of two species – T. orientalis and T. buffeli. Theileria taurotragi and T. mutans generally cause no disease or mild disease, and T. velifera is nonpathogenic. These last three parasites are found mainly in Africa, and overlap in their distributions, complicating the epidemiology of theileriosis in cattle. Theileria lestoquardi, T. luwenshuni and T. uilenbergi affect sheep and goats. Theileria lestoquardi, also transmitted by Hyalomma ticks, is the only species of economic significance infecting small ruminants. East Coast fever/corridor disease (T. parva) is characterised by fever, generalised lymphadenopathy, anorexia, loss of condition and, in some animals, nasal discharge or diarrhoea. Petechiae and ecchymoses may be found on the conjunctiva and oral mucous membranes, and milk yield usually decreases in lactating animals. Terminally ill animals often develop pulmonary oedema, with severe dyspnoea and a frothy nasal discharge. Tropical theileriosis (T. annulata) generally resembles East Coast fever, but these parasites also destroy red blood cells, causing anaemia and, in some cases, jaundice or haemoglobinuria. Petechiae are often found on the mucous membranes, and haemorrhagic diarrhoea may be seen in the late stages. Other species of Theileria tend to be carried asymptomatically, although some can cause anaemia or other clinical signs, especially when there are exacerbating factors such as coinfections. There is no evidence that the species of Theileria found in ruminants affect humans.