An attempt is made to revise comprehensively the systematics of the families, subfamilies, and genera of the Erythraeoidea (Acarina). Its principal aim is to clarify and stabilize the nomenclature of the previously described members of the two families Erythraeidae and Smarididae, both for the adults or nymphs and the larvae. Definitions are given for the superfamily Erythraeoidea (and its relationship to other Trombidiformes discussed), the families, subfamilies, and genera. In a few instances it has been necessary to describe species in order to clarify higher taxa. The early history of the nomenclature of the erythraeoid mites is discussed. The geological history of these mites is considered, as far as scanty knowledge allows. The chaetotaxic nomenclature for the Erythraeoidea and other Trombidiformes is reviewed and revised. An attempt is made to establish a uniform system of setal nomenclature, using mainly descriptive terms of general applicability for these and other mites. The systematic revisions proposed in the past for the Smarididae are critically reviewed. Re-examination of the characters of larval Smarididae, known to be such by experimental rearing, has shown that Grandjean's criterion for the separation of larval Smarididae from Erythraeidae, i.e. the presence of pedal trichobothria in the Smarididae against the absence of these in the Erythraeidae, appears to be the only valid criterion (with a possible reservation in the case of Phanolophus). The genus Erythraeus Latreille, 1806, is restricted to extra-Australian forms, and new genera and subgenera are erected for various Australian Erythraeinae, one of these being Rainbowia, gen. nov., with type species Rainbowia imperator (Hirst, 1928). Rearing experiments correlating Rainbowia imperator with its larva, known previously as Erythraeus oudemansi (Womersley, 1936), are described. Other observations on the biology of this species are recorded. The larva has been found in the Adelaide region to be an ectoparasite on psyllids feeding upon eucalypts, particularly Eucalyptus camaldulensis. The subfamily Callidosomatinae is expanded from its former definition, while the subfamily Balaustiinae is restricted. In the Callidosomatinae the genus Charletonia Oudemans, 1910, is revived, and the genus Caeculisoma Berlese, 1888, is defined for its larval characters. Observations by some earlier students on the biology of the Erythraeoidea are reviewed. Some general biological concepts are considered in relation to the Erythraeoidea. Among the Erythraeoidea the larvae are generally heteromorphous in relation to the postlarval forms. In the Balaustiinae, however, the relationship between the larvae and the eight-legged forms is one nearer homomorphy. The concept of ontogenic precocity or "acceleration" among the Balaustiinae is examined. A glossary is given of the terms used or discussed in the present work.