Molluscs (snails, slugs, clams, mussels, chitons, squid, cuttlefish and octopus) are invertebrate animals that can be found in almost all types of habitats, from the bottom of the oceans to the Alps. They are very diverse, not only in size and shapes, but also in behaviour and in habitat. They are an important food source for birds, fish, mammals and other invertebrates, as well as for humans, and play a key role in the recycling of nutrients. They are good indicators of environmental quality, especially for rivers and lakes and for ancient grassland or marshes. Furthermore, bivalves such as mussels are responsible for cleaning large quantities of water. Terrestrial snails are often known as garden pests. However, the pest species represent only a minority of the species existing in Europe.
All freshwater mollusc species native to Europe or naturalised before AD 1500 were included in the assessment, except two that only have a marginal occurrence in Europe. In total, 854 species were considered. Freshwater molluscs fall into two main groups, the Bivalves and the Gastropods, the latter being much more numerous.
All terrestrial snails belong to the group Gastropods. It was not considered possible, within the timescale allowed, to assess and evaluate all c. 2,700 terrestrial mollusc species listed at the time, so two superfamilies were prioritised, which contained many of the European Habitats Directive species, have a wide European distribution and cover a range of different habitats. Additional families were included such as the semi-slugs (Vitrinidae) and some "Prosobranch" families to provide a different phyletic range. Within the families selected, all the species were included, amounting to 1,233 species.