The genus Achias Fabricius occurs in New Guinea, northern Australia, and some islands adjacent to these areas. Evidence is presented as to the disputed publication dates for Hendel's two significant reviews of the Platystomatidae as these affect nomenclature. Available information on Achias biology is summarised, but includes no larval records. A method of estimating the approximate number of undiscovered species is presented, and indicates that the register of Achias species is very incomplete. A description of the genus Achias is given. Generic synonyms and means of generic determination are discussed. Certain taxonomic characters are discussed and these are related to taxonomic placement of the genus, informal infrageneric groupings of species, and species delimitation. A key to all recognised species is given. In total, 67 species (listed in index) are described as new. No synonyms among nominal species of Achias are recognised. Four apparent species represented by inadequate available material are assigned provisional numbers. An etymology of Achias nomenclature is appended.