Oligochaeta sensu stricto, namely clitellates exclusive of branchiobdellids and leeches, occur in marine, estuarine, freshwater and terrestrial environs. About one-third of the almost 5,000 valid species described to date is aquatic. With the exception of some earthworm-like genera (the “megadriles”), aquatic oligochaetes are usually small, ranging from 1 mm to a few centimetres in length (the “microdriles”). Although predominantly terrestrial, 4 of the 14 described megadrile families include species that occur in aquatic or semi-aquatic habitats. The microdriles are fully aquatic, with the exception of the primarily terrestrial family Enchytraeidae, and comprise 13 families. About 1,700 valid species of aquatic oligochaetes are known to date; of these, about 1,100 are freshwater. The most speciose group is the Tubificidae with over 1,000 described species including 582 being considered as freshwater inhabitants. No fewer than 60 species of megadriles are also considered aquatic. Recent years have seen a continuous increase in the number of described species, so that any estimate of the proportion of known freshwater oligochaete species to unknown species would be very imprecise. Molecular studies have recently confirmed the long suspected paraphyly of the Oligochaeta if the group does not include branchiobdellids and leeches, so that Clitellata has become synonymous with “Oligochaeta”. The family Capilloventridae has been recently shown to represent a basal clade of Clitellata, supporting an aquatic (freshwater?) origin of the clitellates. In contrast, the adaptation to freshwater of the aquamegadriles is most likely secondary. The Palaearctic region supports the most abundant and diverse freshwater oligochaete fauna, with more than 600 valid species described to date; 80% of these are considered endemic. However, it is likely that the apparent concentration of genera and species in the Northern Hemisphere is biassed given the relatively late and still limited interest in the oligochaete fauna of the Southern Hemisphere. Ancient lakes, as well as ground waters, are important centres of endemicity but, except for Lake Baikal, they represent important knowledge gaps. Aquatic oligochaetes perform ecological functions and roles with potentially important repercussions for human health issues. These ecological values of oligochaetes include their importance in aquatic food chains; their impact on sediment structure and water-sediment exchanges; their long history of use in pollution monitoring and assessment; their potential to reduce sludge volumes in sewage treatment systems; and their role as intermediate host for several myxozoan parasites of fishes, including commercially exploited species.