This paper comprises the first revision of all worldwide described Glyceridae. Under the application of light and scanning electron microscopy most of the 172 species and subspecies of the glycerids are examined by their type material. Three of these species (? Glycera baltica (Eisenack 1939), ? G. glaucopsammensis Charletta & Boyer 1974, ? G. pilicae SZANIAWSKI 1974) represent fossil taxa and are therefore not further investigated. The descriptions and drawings of three other taxa ( Lumbricus capitatus O. Fabricius 1780, Nereis filigera Blainville 1825, Nereis (Nephtys) hirsuta Dalyell 1853), which have been referred to as glycerids, are clearly shown to belong to other families (Capitellidae, Cirratulidae, Nephtyidae). Glycera viridescens Stimpson 1854 seems to be a goniadid, whereas the taxon Pseudolacydonia caeca Rullier 1964 is a synonym for Glycerella magellanica (McIntosh 1885). Altogether, only 42 species (36 Glycera, 1 Glycerella and 5 Hemipodia ) of the 166 recent taxa are considered to be valid species, the others being synonyms or nomina dubia and nomina nuda. More than 4000 specimens from all over the world were studied and measured, and the 42 valid species are described and illustrated in detail. A list of all taxa together with their synonyms and an identification key for all Glyceridae known to this date are also presented. Addionally all available biological and ecological data from the literature are listed. The results of the cladistic analysis support a sister group relationship between Goniadidae and Glyceridae. Both taxa are monophyletic as well as the three genera Glycera, Glycerella and Hemipodia within the glycerids.