The history of frankincense, starting in ancient Egypt, and the frankincense trees, what is now the genus Boswellia Roxb., is briefly outlined. General remarks are given on phylogeny and dating of the genus, previous taxonomic treatments, authorship and etymology, chromosome number, cotyledons and hypocotyl, cliff-dwelling versus ground-dwelling species, bark, stipules, breeding system and pollination, fruits and dispersal, hybrids, chemistry and uses, conservation, and harvesting and processing of frankincense. According to previously published phylogenies, Boswellia is sister to the Asian genus Garuga Roxb., and the crown group dates back to the mid Miocene. Twenty-four species are recognized in Boswellia, distributed from Mali and Burkina Faso in the west to India in the east, and from Oman and north-central India in the north to northeastern Tanzania in the south. Most species are found in the Horn of Africa region, with the highest concentration (11 species) in the Socotra archipelago (Yemen). A key to the species of Boswellia is provided, as well as distribution maps, illustrations, descriptions, notes on etymology, distribution and habitat, vernacular names and uses, IUCN Red List assessments, taxonomic remarks, and lists of collections, for all species. Boswellia aspleniifolia (Balf.f.) Thulin, comb. nov., based on Odina aspleniifolia Balf.f., B. samhaensis Thulin & Scholte, sp. nov., and B. scopulorum Thulin, sp. nov., are published, all from the Socotra archipelago. The name Boswellia “hesperia” is provisionally proposed for an apparently new species from Socotra, of which only photographs have been available. Fifteen names are lectotypified, and a neotype is designated for B. papyrifera (Delile) Hochst.
