Ph.D. / The complexity of Combretaceae and lack of information on phylogenetic relationships within the family led me to explore relationships between and within genera of Combretaceae by means of combined analyses of plastid (rbcL, psaA-ycf3 spacer, and psbA-trnH spacer) and nuclear ribosomal ITS sequences for all but three of the 17 genera in Combretaceae. The current classification of the family into two subfamilies, Strephonematoideae and Combretoideae, is corroborated. Within Combretoideae, division into two tribes, Laguncularieae and Combreteae, is strongly supported. Within Combreteae subtribe Terminaliinae relationships between genera are largely unresolved. Terminalia is not supported as monophyletic, and two groups were identified, one containing mainly African species and another of mostly Asian species. Pteleopsis, Buchenavia and Anogeissus are embedded within Terminalia, and I suggest that all genera of Terminialiinae with the exception of Conocarpus should be included into an expanded Terminalia. Within subtribe Combretinae, a clade formed by the two monotypic genera Guiera and Calycopteris is sister to the rest of the subtribe. Within Combretinae, groupings are consistent with recent results based on morphological data. Combretum is currently divided into three subgenera: Apethalanthum, Cacoucia and Combretum. The two last were included in this study and supported as monophyletic. Meiostemon is sister to subgenus Combretum, and Quisqualis is embedded within subgenus Cacoucia. I recommend that subgenus Combretum should be expanded to include Meiostemon and subgenus Cacoucia to include Quisqualis. The sectional classification within Combretum proposed in earlier morphological studies is confirmed except for the exclusion of Combretum imberbe from section Hypocrateropsis into a separate and monotypic section and the inclusion of Combretum zeyheri (section Spathulipetala) within section Macrostigmatea. In order to accommodate Combretum imberbe a new section is suggested which is formally described in Maurin et al. (2010). The reinstatement of previously recognised sections Grandiflora and Trichopetala, both of which had been sunk into subgenus Cacoucia section Poivrea, is also suggested. In this study two new species from South Africa are presented: Combretum nwambiyana O.Maurin, M.Jordaan & A.E.van Wyk and Combretum stylesii Maurin, Jordaan & A.E.van Wyk. The formal description of these two new species will be done in regular journals. Divergence time estimates using a Bayesian MCMC approach implemented in BEAST suggested a crown date for Combretaceae around 82.6 mya with the two subtribes Combretinae and Terminaliinae splitting at the end of the Late Cretaceous during a period marked by a number of mass extinctions both in plants and animals. The Miocene and Pliocene are characterised by constant speciation with many clades within Combretaceae succesfully adapting to savanna vegetation and diversifying within it.