The taxonomy of the family Megaloeporaceae (Lecanorales, lichenised ascomycetes) has been revised basing on an examination of morphology, anatomy and chemistry of thallus and apothecia, and of apothecium ontogeny. The Megalosporaceae are mainly characterised by features of the apothecium ontogeny, by thallus- and apothecium-chemistry, and by a range of spore types. Three genera have been distinguished within this family, Austroblastenia (gen. nov.) with two species, occuring in Australia and New Zealand, Megaloblastenia (gen. nov.) with two species, occuring in Australia, New Zealand and southern South America, and Megalospora, with twenty-five species, concentrated in Australasia, but with representatives throughout the tropical and warmtemperate zones of the world. The delimitation of these genera is based on spore structures. The species delimitations are mainly based on features of the epithecium pigmentation and granulation, spore structure and size, hymenium size and thallus chemistry. Species are defined as entities characterised by at least two independent differentiating characters. Entities with only a single such character (e.g. "chemospecies" or "secondary species") have been treated below specific rank or left unnamed. The delimitation of the genus Megalospora has been changed. Several species had to be transferred to other genera, notably Catinaria, while on the other hand several Bombyliospora species, including its type, as well as a few species with muriform spores, have been included in Megalospora. In all 11 species and 3 subspecies are newly described. Nearly all members of the family inhabit humid, cool forests, often cloud forest in tropical mountains. Basing on a cladistic analysis the ancestor of the family is supposed to have had bicellular spores with thick septa, which supports a relation with the Buelliinae. Most of the evolution of the family is supposed to have taken place on Gondwana-land, from which main migration routes have brought species into tropical Africa and America, into tropical and eastern Asia, and into southern South America.