The genus Halacarellus Viets, 1927 holds 50 species, and this number equals 4% of all halacarid species known to date. The genus is spread in all oceans, and a few species are even known from freshwater. The genus includes species which live on and amongst surface structures, algae, colonial organisms, and macrofauna and in interstices and microcaverns. The North Atlantic species, the ones best studied, live in a wide range of habitats, depths, salinities, and temperatures. In the present paper, morphological characters of each species, geographical records, and ecological data are summarized in tables. The table of character states is meant both for identification of species and to get an idea of shared characters and hence natural lineages within the genus. Halacarellus is rare in tropical but abundant in cold-temperate and polar waters, both in number of species and individuals. Most records are from the lower tidal and shelf zone, only two from beyond 1000 m depth. The North Atlantic species differ in their morphological character states from those of the Southern Oceans. Each region contains other natural species groups; none of the outlined groups is present in both the northern and southern hemispheres. The morphological differences do not support a splitting of the genus Halacarellus.