The center of diversity for the family is the western Palearctic, especially the Iberian peninsula (Wiktor 1996).
About 35 species are in the genera Arion, Geomalacus, and Letourneuxia (Barker & Efford 2004).
Native throughout Europe and north to Scandinavia, N Africa down to the Sahara, with some species in Asia east to the Urals (and Siberia); major speciation center in Iberian Peninsula (De Winter 1986; Wiktor et al. 2000); not in the Caucasus or Black Sea region (Wiktor 1983).
For its defense, an arionid slug will retract its head, become hemispherical, and stick down to the substrate. Many Arion species have extra colored pigment as a deterent in the slime that they produce when perturbed (Rollo & Wellington 1979).
Consumes plant, fungus and sometimes feces, carrion (Herbert 1997).