Trigonostemon Blume is a plant genus in the family Euphorbiaceae comprising 59 species. These plants are small trees or shrubs growing in the lowland rainforests in Southeast Asia and adjacent areas. The small unisexual flowers with colourful petals and the 3 or 5 united stamens are typical characters of the genus. The present thesis studies four aspects of the genus: the taxonomy, pollen morphology, molecular phylogeny and historical biogeography. The species delimitation has been optimised in the taxonomic revisions and the species complexes can now be clearly recognised by morphological characters. Palynological studies reveal strong correlations between pollen and macromorphology in Trigonostemon and these support the infrageneric classification of the genus. Molecular phylogenetic analyses have demonstrated that Trigonostemon and Dimorphocalyx Thwaites are two monophyletic groups and are not closely related, although they are similar in morphology. Furthermore, Trigonostemon is divided into four sections based on molecular and (pollen) morphological data. Finally, the thesis concludes with the historical biogeography of Trigonostemon: it probably originated on Southeast Asian mainland, one lineage later radiated to the Malay Peninsula, and dispersed further eastwards and diversified before the genus reached its current distribution.