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This study is based on a study at Trinity College, Dublin, for a doctoral degree. It constitutes a taxonomic revision of the genus Asterella in Eurasia. Asterella belongs to the family Aytoniaceae in the Complex Thalloid Liverworts (Marchantiales). It is poorly understood taxonomically and a worldwide revision of the c. 158 described species is badly needed. Its gametophyte is morphologically complex and rich in qualitative characters; the sporophyte is highly reduced but shows great diversity in spore ornamentation patterns. Marchantiales are among the earliest land plants but relationships and phylogeny are poorly understood. A morphological taxonomic revision of available Eurasian material is presented, based on both field and herbarium studies. Cytological studies yield little useful data. Subgenera and sections are defined. Sixteen species are accepted and described in detail, along with keys to subgenera and species (including regional keys), full synonymy and typification, including designation of many new lectotypes. Spore ornamentation proved vital in identifying old fragmentary type specimens. For each species geography and ecology are summarised. Where appropriate, detailed notes on typification, nomenclature, synonymy, era, are given. Excluded taxa are listed and discussed. The revision demonstrates a major advance in knowledge of Eurasian Asterella: extensive reduction in names due to past over-description; the existence of two undescribed species; the importance of characters such as sexual condition and spore ornamentation, and the plasticity of many quantitative characters; clarification of phytogeography with a number of localised endemics and several very widespread taxa.