A taxonomic revision of the Australian members of trentepohlioid, ecolumellate thelotremataceaen Graphidaceae (Ostropales, lichenized Ascomycota) is presented. This species-rich group of crustose, predominantly tropical and subtropical lichens has been neglected for a long time, and is still poorly known. Particularly the studies for Australia are highly fragmentary. Approximately 4000 specimens, including a total of c. 500 type specimens, herbaria collections, and almost 1000 samples collected during two field trips, were examined morphologically, anatomically, and chemically. A total of 103 species of 13 genera (including one newly described genus) could be identified. 33 species are new records for Australia, and 20 species were described new to science. Of the currently listed 71 names of this group for Australia, 50 are accepted as valid species. The remaining names and species, listed in an appendix, proved to belong to other genera, be synonymous to other taxa or falsely identified collections. 21 species (including species not known for Australia) are newly combined and several new synonyms are proposed. In the taxonomic part an introduction is provided, including a historical overview of the development of generic concepts and delimitations within the treated group. This is followed by a brief presentation of earlier taxonomic studies and collections of thelotrematoid lichens in Australia. It also provides a general treatment of the climatic and biogeographical conditions of the continent. Further, the main characters are described, including anatomical, morphological and chemical features. The ecology and distribution of thelotrematoid lichens are discussed. All treated species are described and illustrated, their distribution is mapped, and a key for the identification provided. An overview of the distinguishing characters between genera is presented in a table. To illuminate the phylogenetic relationships within this group, three phylogentic analyses were carried out using partial sequences of nuclear and mitochondrial ribosomal DNA loci. The first analysis employed a combined dataset of the mitochondrial SSU (mtSSU) and the nuclear LSU (nuLSU) of 105 species of Ascomycetes. As a result, it could be shown that the hitherto separted two families of Ostropales, Graphidaceae and Thelotremataceae, are not monophyletic, but form several lineages within one strongly supported monophyletic group. Consequently, Thelotremataceae is reduced to synonymy with Graphidaceae. The second and third analyses were carried out using a single dataset of nuLSU and mtSSU sequences respectively. In the nuLSU analysis 40 specimens of 38 species were included, in the mtSSU analysis 50 specimens of 25 species were examined. These analyses showed that all genera of thelotrematoid lichens included in the study are para- or polyphyletic, with the sole exception of Diploschistes, and that the present generic concept within Graphidaceae needs revision. The new genus Melanotopelia is introduced based on molecular and morphological evidence.