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Tropical rain forests are the most biologically diverse of terrestrial biomes. A few species-rich genera contribute disproportionately to this diversity and yet have often been neglected taxonomically because their sheer size makes them so challenging. Understanding the causes and mechanisms of these lineage diversifications is, however, central to understanding the evolution of plant diversity and for effectively planning its conservation. To realise the potential of these large genera, timecalibrated, phylogenetic studies are required to place the species in their evolutionary and biogeographic context, effectively plan future taxonomic work and to investigate the patterns and processes of their diversification. Cyrtandra is the largest genus in the Gesneriaceae with over 800 species of herbs, shrubs and occasionally trees, and is one of the fifty largest plant genera in the world. It is found from the Nicobar Islands in the Indian Ocean, throughout Malesia, in Taiwan and the southern Japanese islands, in northern Australia and east across Polynesia to Hawaii. It is found in forest, from sea level to 3000m. Taxonomic work is required throughout the distribution of Cyrtandra but primarily in its centres of diversity in Southeast Asia, on islands such as Sulawesi and Borneo. There is no functional infrageneric classification in the genus greatly increasing the difficulty of working with the group and hampering the planning of future taxonomic work. This study includes a taxonomic revision of Cyrtandra on the Indonesian island of Sulawesi which has resulted in the recognition of 40 species. Of these, five species are highlighted as new in this thesis: Cyrtandra longistamina H.J.Atkins & Karton. ined., Cyrtandra balgooyi H.J.Atkins & Karton. ined. Cyrtandra flavomaculata H.J.Atkins & Karton. ined., Cyrtandra parvicalyx H.J.Atkins & Karton. ined. and Cyrtandra sopuensis H.J.Atkins & Karton. ined. Eleven species are lectotypified and three names are placed in to synonymy. Provisional conservation assessments show that two of these species are Critically Endanagered, one is Endangered, thirteen are Vulnerable, one is Near Threatened and 22 are of Least concern. Generation of a well-sampled phylogeny of Cyrtandra across Southeast Asia and the Pacific and optimisation of eight macromorphological characters, as part of the current study, has allowed the development of a strategy for tackling this large genus taxonomically in the future in the absence of a functional infrageneric system. The poor diagnosability of the six major clades, the homoplasy of the macromorphological characters, combined with some degree of floristic exchange between islands, showed that a single approach, either based on geography, morphology or molecular data, would not be satisfactory for Cyrtandra and, similar to other large genera, an integrative method would be most successful. I assessed Clark et al. (2013)’s concept of a phylogenetically informed revision of a geographic area on the Cyrtandra of Sulawesi and Borneo and found it to be a promising approach. I now have a key to fourteen lineages on Borneo that can form the basis for taxonomic research on the 200 plus species on that island in the future. Molecular dating, ancestral range estimation and biogeographic stochastic mapping included here have allowed the examination of the geo-temporal patterns and evolution of southeast Asian Cyrtandra and to show that the great diversity of Cyrtandra seen in the Malesian region results from a recent radiation, with most speciation taking place in the last five million years. Borneo was recovered as the most likely ancestral range of the genus, with the current distribution of species resulting from a west to east migration across Malesia that corresponds with island emergence and mountain building. High levels of floristic exchange between the islands on the Sunda shelf and an important role for the Philippines as a stepping stone to Wallacea and New Guinea are uncovered. These patterns underlie much of the plant diversity in the region and form an emerging paradigm in Southeast Asian plant biogeography. Novel observations on speciation dynamics for southeast Asian Cyrtandra made here include (i) the absence of hybridisation, possiby due to the role of pre-zygotic barriers associated with high levels of floral divergence in sympatric populations in southeast Asia; (ii) a remarkably stable genome with no variation in chromosome numbers recorded across its distribution, including the Pacific. These two observations are notable as they are in contrast to other large genera such as Begonia, Inga and Solanum where hybridisation and ploidy changes are common. The dense phylogenetic sampling for interspecific and intraspecific diversity, combined with new trait and geographical datasets have provided much insight into the origin of the diversity of Cyrtandra. Future work can focus on the taxonomy of Cyrtandra on the large islands of Borneo and New Guinea and on the genomic architecture of species differences.