Life histories of species of Plasmodium, Hepatocyslis, Leucocytozoon and Haemoproteus are reviewed. The last two genera are considered in more detail than the others and some data from recent experiments on Leucocytozoon are given to support the concept of the life cycle that is presented. The importance of the feeding habits and host preferences of the vectors is noted as well as the specificity of the parasites in determining their occurrence. Plasmodium and Hepatocystis are transmitted by mosquitoes and biting midges respectively; the latter as well as hippoboscids are vectors of Haemoproteus while simuliids and biting midges transmit species of Leucocytozoon . These differences together with others in the life cycles suggest that revisions in classifications may be desirable. The need is emphasized for further research on the life cycles of species in all genera and for more details on species that are better known.