Over recent years, there has been a growing interest in bacteria from the genus Anaplasma, especially the species A. marginale, A. ovis and A. phagocytophilum. It is connected with the pathogenic activity of these bacteria in farm animals, and also, though to a lesser degree, in humans. Anaplasmosis, a disease caused by various species of anaplasma, is an especially important issue for animal breeders. The main vectors of the Anaplasma bacteria are ticks, common arachnida occurring everywhere in the world, especially the genera Ixodes, Dermacentor, Rhipicephalus and Amblyomma. The genus Anaplasma includes obligate intracellular bacteria, parasitizing in the vacuoles of cells in eukaryotic hosts. A. marginale, A. centrale, A. ovis and A. bovis are obligate intracellular bacteria parasitizing in erythrocytes and monocytes of higher vertebrates, mostly ruminants. A. platys is mainly a pathogen of canines (displaying tropism to thrombocytes) and the species A. phagocytophilum (displaying tropism to granulocytes) is pathogenic to people and domestic animals. In this paper we present characteristics and differentiation of six species of the genus Anaplasma and their vectors in the world.