A world wide revision of the genera Mitragyna s.l. and Uncaria both placed in the subtribe Mitragyninae Havil. of the tribe Cinchoneae, with a general discussion on the affinities of the genera, the growth form and architecture. In Mitragyna 10 species and in Uncaria 34 species are recognized; 4 new forms of Uncaria are described. There are keys to the genera and species. The species occurring in continental Asia but not in Malesia have complete synonymy and descriptions whilst the African, American, and Malesian species are treated in an abbreviated form. A complete list of scientific names is included.