The aims of this study are: 1) to reconstruct a skeleton phylogeny of the orchid genus Coelogyne and allied genera based on molecular and morphological characters; 2) to incorporate this phylogeny into a phylogenetic classification of the Coelogyninae; 3) to provide taxonomic revisions of a selection of species groups of Coelogyne. Coelogyne comprises over 200 species distributed throughout southeast Asia with main centers of diversity in Borneo, Sumatra and the Himalayas. Most species are epiphytes and occur in primary forests. They have a fairly large number of mediumsized to large flowers with delicate colours and a sweet scent, which are pollinated by bees, beetles or wasps. The genus is placed in subtribe Coelogyninae (subfamily Epidendroideae) together with 15 other genera with a total of approximately 550 species. The subtribe is characterised by sympodial growth, pseudobulbs of one internode, terminal inflorescences, a winged column and massive caudicles. Separate maximum parsimony analyses of RFLPs, matK and nuclear rDNA ITS sequences, macromorphological and anatomical data collected for 27 Coelogyne species and 13 representatives of related genera produce largely congruent results. A total evidence analysis indicates that Coelogyninae are monophyletic and diverged early into three major clades. Clade I comprises species of Coelogyne sect. Coelogyne, subgenus Cyathogyne, sect. Rigidiformes, Tomentosae, Veitchiae and Verrucosae, from which Bracisepalum, Chelonistele, Dendrochilum, Entomophobia, Geesinkorchis and Nabaluia split off. Synapomorphies for this group of species are the more than 15 flowers per inflorescence, presence of sterile bracts on the rachis and presence of hairs on the ovary. Elongate trichomes with acute top on the leaf surface, synanthous inflorescences, presence of sterile bracts at the base of the rachis, simultaneously opening flowers, persistent floral bracts, ovate-oblong petals, and hairy sepals are present in the majority of taxa in this clade. Clade II subsequently diverged into species of Neogyna and Pholidota nested within species of Coelogyne sect. Bicellae, Brachypterae, Elatae, Flaccidae, Fuliginosae, Hologyne, Lentiginosae, Longifoliae, Moniliformes, Ptychogyne and Speciosae. Synapomorphies for this group are the caducous floral bracts, glabrous ovaries, linear petals and a relatively low number of morphologically diverse keels on the hypochile. Hysteranthous inflorescences, less than 15 flowers per inflorescence, intermediate-sized flowers and a relatively low number of keels on the epichile are present in the majority of taxa in this clade. Clade III consists of species of Pleione and is characterised by short-living pseudobulbs, a lack of stegmata in all sclerenchymatous tissues, a hypochile without lateral lobes and an epichile apex with fimbriate margin. The traditional circumscription of Coelogyne is not supported by the total evidence phylogeny presented here and should be abandoned. A redefinition of the genus is suggested by including Neogyna and Pholidota and removing the species of Coelogyne sect. Coelogyne (in part), Cyathogyne, Tomentosae, Rigidiformes, Veitchiae and Verrucosae. A formal proposal for the creation of a new genus for these species is not made yet, as most internal nodes of the total evidence tree are only poorly supported and need a larger taxon sampling and data from more variable genes. 4 The number of subgeneric groups recognised by various authors in Coelogyne varies between 5 and 23, which is mainly due to the relative lack of morphological characters available to define groups of species. Of the 17 sections sampled in Coelogyne, just three (with only two sampled species each) form strongly supported monophyletic groups in the total evidence analysis: sect. Longifoliae, Moniliformes and Verrucosae. This is consistent with the clear morphological synapomorphies that characterise those sections. Monophyly of Coelogyne sect. Flaccidae and Tomentosae is weakly supported, which is in accordance with the few and not unique synapomorphies that define these sections. Coelogyne sect. Coelogyne and sect. Elatae are clearly paraphyletic. This was already expected as the morphological diversity in both sections is high. A well-supported subset of species is formed by C. fimbriata (sect. Fuliginosae) and C. stricta (sect. Elatae), which share the presence of sterile bracts on the base of the scape. To investigate whether this clade warrants the status of a new section, a much larger sampling within Coelogyne is needed. The species sampled of subgenus Bicellae, Cyathogyne, Hologyne and Ptychogyne seem well nested within several sections of Coelogyne and do not warrant the status of subgenus. Several of the traditionally used floral traits for (sub)generic and sectional delimitation within Coelogyninae and Coelogyne (the ‘key’characters) were plotted on the total evidence tree. Inflorescence type, number of flowers per inflorescence, persistence of floral bracts, presence of sterile bracts on the rachis, ovary indumentum, petal shape, presence and shape of lateral lobes of hypochile, number of keels on the epichile and presence of a fimbriate margin on the epichile appear to be good synapomorphies for major clades in Coelogyninae and Coelogyne. The number of leaves per pseudobulb, size of the flowers, shape of the lip base and petals and presence of stelidia and calli on the lip show many reversals and appear not to be phylogenetically informative. With the phylogenetic boundaries of the total evidence analysis as a reference, a start with a taxonomic treatment of the whole genus is made by revisions of three different groups of species in Coelogyne. An integrated phylogenetic analysis of morphological and molecular characters is performed for the 16 species of sect. Speciosae and 8 species of sect. Verrucosae to check monophyly and study interspecific relationships, whereas a complex of the closely related species of sect. Fuliginosae is resolved with a phenetic analysis using morphological characters. The last three chapters of this thesis contain descriptions of all species (including three new ones), synonyms, photographs, drawings, distribution maps and identification keys.