Species of Polylepis are shrubs or trees native to the mid- and high-elevation tropical Andes. Some species of Polylepis form woodlands growing well above normal treeline within grass and scrub associations at elevations over 5000 m. Consequently, Polylepis appears to be the highest natural occurring arborescent angiosperm genus in the world. The physiological basis allowing such high altitude growth is not understood, but evidence indicates that woodlands at these elevations are restricted to microsites where ecological conditions are similar to those of lower altitudes. The genus is distinct from other rosaceous members of the tribe Sanguisorbeae and is interpreted here as consisting of three species groups that appear to have spread independently north and south along the Andean chain. Because of the patchy distributions of populations and disturbances by man, species are often polytopic. As a result, numerous forms have been described as distinct taxa in the past. In addition, hybridization appears to occur between species making circumscription of some taxa difficult. In this revision, evidence from studies of gross morphology, leaflet anatomy, pollen morphology, and field observations of the autecology of various populations were used to ascertain the number of species within the genus and their relationships to one another. Data from these sources indicate that Polylepis should be considered as consisting of 15 species, one of which is first described in this treatment. A key for the identification of the species is provided. Each species is illustrated, discussed, and its modern distribution mapped.