A taxonomic revision of the African genera of Costaceae (Costus and Paracostus) is given. Within the genus Costus 24 species are recognized, 8 of which are here described as new and one is given a new name. Included are chapters on the history of the taxonomy of the family, morphology, flower biology, pollination, dispersal, distribution, ecology, phylogeny and molecular studies and conservation. The species treatments include descriptions, full synonymy, geographical and ecological notes and taxonomic notes. For all species distribution maps are provided. A complete identification list with all exsiccatae studied and an index to scientific names is included at the end.