A taxonomic monograph of the ascomycete genus Taeniolella (asexual dematiaceous hyphomycetes, sexual morphs unknown) is provided. Recent phylogenetic analyses demonstrated the polyphyly of this genus. The type species of Taeniolella pertains to the Kirschsteiniotheliaceae within Dothideomycetes, while other saprobic species clustered far away within Sordariomycetes, Savoryellaceae s. lat., and Lindgomycetaceae, whereas lichenicolous species belong to a monophyletic clade that represents the order Asterotexiales, but for most species assigned to Taeniolella sequence data and phylogenetic analyses are not yet available. The main focus of the present taxonomic study was on a revision of the lichenicolous Taeniolella species. Since the currently available phylogenetic analyses do not allow final taxonomic conclusions at generic rank, the exclusion of lichenicolous species from Taeniolella s. lat. has been postponed pending a broader sampling and more phylogenetic data of allied ascomycete genera within the order Asterotexiales. For the interim, Taeniolella s. lat., including lichenicolous and saprobic species, is maintained. The taxonomic background, history, generic description and discrimination from morphologically confusable genera, phylogeny, biology, host range and distribution, and species concept of Taeniolella species are briefly outlined and discussed. Keys to the species of Taeniolella divided by ecological groups (lichenicolous taxa, saprobic taxa) are provided, supplemented by a tabular key to lichenicolous species based on host (lichen) families and genera. Twenty-nine lichenicolous species and a Taeniolella sp. (putative asexual morph of Sphaerellothecium thamnoliae) as well as 16 saprobic species are described in detail and illustrated by drawings, macroscopic photographs, light microscopic and SEM micrographs, including six new lichenicolous species (T. arctoparmeliae on Arctoparmelia separata, T. lecanoricola on Lecanora rupicola, T. thelotrematis on Thelotrema, T. umbilicariae and T. umbilicariicola on Umbilicaria, T. weberi on Thelotrema weberi), three new saprobic species (T. filamentosa on Salix, T. ravenelii on Quercus, T. stilbosporoides on Salix caprea), and one new combination, T. arthoniae. Most saprobic Taeniolella species are wood-inhabiting (on bark, decorticated trunks and twigs, rotten wood), whereas lichenicolous species grow on thalli and fruiting bodies (mostly apothecia) of lichens, mostly without causing any evident damage, but they are nevertheless confined to their host lichens, or they are obviously pathogenic and cause either disease of the thalli (e.g., Taeniolella chrysothricis and T. delicata) or at least thallus discolorations or necroses (e.g., T. christiansenii, T. chrysothricis, T. cladinicola, T. pseudocyphellariae, and T. strictae). Taeniolella atricerebrina and T. rolfii induce the formation of distinct galls. The range of micro-morphological traits for taxonomic purposes is limited in Taeniolella species, but size, shape and septation of conidiophores and conidia, including surface ornamentation, provided basic characters. Mycelium, stromata and arrangement of conidiophores are less important for the differentiation of species. Lichenicolous species are widespread on a wide range of lichens, with a focus in the northern hemisphere, mainly in northern temperate regions, including arctic-subartic habitats (18 species, i.e., 62 % of the lichenicolous species). Eleven lichenicolous species, e.g., T. pseudocyphellariae, T. santessonii, T. thelotrematis, T. umbilicariae, are also known from collections in non-temperate Asia, Australia and South America (38 % of the species). Most collections deposited in herbaria are from northern temperate to arctic-subarctic regions, which may reflect activities of lichenologists and mycologist dealing with lichenicolous fungi in general and Taeniolella in particular. Most lichenicolous Taeniolella species are confined to hosts of a single lichen genus or few closely allied genera (26 species, i.e., 97 % of the lichenicolous species), but only three species, T. delicata, T. punctata, and T. verrucosa, have wider hosts ranges. Excluded, doubtful and insufficiently known species assigned to Taeniolella are listed at the end, discussed, described and in some cases illustrated, including Talpapellis beschiana comb. nov. (≡ Taeniolella beschiana), Corynespora laevistipitata (≡ Taeniolella laevistipitata), Stanjehughesia lignicola comb. nov. (≡ Taeniolella lignicola), Sterigmatobotrys rudis (≡ Taeniolella rudis), and Taeniolina scripta (≡ Taeniolella scripta).