The taxonomic classification of Babesia species places them in the phylum Apicomplexa, the order Piroplasmida and the family Babesiidae. This classification was originally based on morphological characteristics of the intraerythrocytic parasites (merozoites and trophozoites) and other life cycle observations; however, the traditional methods of classification have been replaced by molecular genetic techniques, as phenotypic features alone are not sufficient for species differentiation. Molecular phylogenetic analysis has been useful not only for defining the relationships between individual Babesia species, but also for further elucidating the association between Babesia and closely related piroplasms such as Theileria species and Cytauxzoon species. The erythrocytic stages of all three genera are similar, yet they are differentiated phylogenetically and by the presence of distinct exoerythrocytic life cycle stages within the vertebrate host for Theileria and Cytauxzoon and by transovarial transmission, which is a feature of Babesia species.