This thesis addresses three aspects of the systematics of Thomandersia. Firstly, a taxonomic account of Thomandersia is provided. Secondly, the anatomy of Thomandersia is described. Finally, the phylogenetic position of Thomandersia in Lamiales is investigated and the potential of morphological and molecular datasets to resolve relationships in Lamiales is assessed. Six species of Thomandersia are recognised, based upon previous taxonomic treatments, an assessment of variability within the genus, and a rationale for describing infrageneric taxa. An improved key to species, species descriptions, distribution maps and illustrations are presented. A preliminary analysis of molecular sequence data suggests that previous, morphology-based classifications of Thomandersia in Acanthaceae do not reflect phylogenetic relationships. The floral and wood anatomy of Thomandersia are reexamined in this light and discussed in the context of Lamiales. Unique characters of Thomandersia include a particularly large calycine nectary, expanded placentas, irregular corolla aestivation, and spherical, scaly seeds. Thomandersia is found to share anatomical features with all other Lamiales investigated. Detailed anatomical studies of retinacula, a character shared with Acanthaceae, and extrafloral nectaries, which may be homologous to nectaries in other Lamiales, are presented. The majority of anatomical characters are homoplastically-distributed in Lamiales and therefore of little use for diagnosing monophyletic groups. Morphological and molecular phylogenetic investigations support the resurrection of Thomandersiaceae, a family of isolated phylogenetic position with a large number of molecular and morphological autapomorphies. At the same time, the investigations highlight problems in resolving interfamilial relationships in Lamiales. The ultimate aim of phylogenetic analysis in Lamiales is an accurately-resolved tree with stronglysupported relationships between all taxa. Morphological characters are of limited use in achieving this goal: when a large morphological matrix is combined with molecular data only small, poorly-supported increases in resolution are achieved. The increasing availability of molecular sequence data is expected to make it possible, in the near future, to resolve an accurate, supported phylogeny of Lamiales and elucidate the sister group relationships of isolated taxa such as Thomandersiaceae.