Leeches (Hirudinea) constitute a relatively small monophyletic group of highly specialized annelids, but may play important roles as invertebrate predators in freshwater, while others are infamous for their ectoparasitic bloodsucking. About 15% of the 680 described species are marine and slightly less have switched to terrestrial life; the rest are freshwater, divided among 91 genera. They are globally distributed on all continents except Antarctica, reaching the highest diversity in the Holarctic region with one-half of all continental species. Known areas of local endemism are the ancient Siberian lake Bajkal and lake Ohrid (about 10 species each) on the Balkan Peninsula, which is an endemicity area in itself. A small number of sanguivorous species known as “medicinal leeches” have played an important role in traditional and modern medicine, most noticeably four Hirudo spp. from the Western Palearctic.