The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to large. There are many thousands of species of sea snails and sea slugs, as well as freshwater snails, freshwater limpets, land snails and land slugs.

The class Gastropoda contains a vast amount of named species, second only to the insects in overall number. They have an extraordinary diversification of habitats. Representatives live in gardens, woodland, deserts, and on mountains, in estuaries, mudflats, the various intertidal zones, in the abyssal depths of the oceans including the hydrothermal vents, and numerous other ecological niches, including parasitic ones.

The marine shelled species of gastropod include edible species such as abalone, conches, periwinkles, whelks, and numerous other sea snails that produce seashells that are coiled in the adult stage. In a number of families of species, such as all the various limpets, the shell is coiled only in the larval stage, and is a simple conical structure after that.

The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian. By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic Era are too poorly preserved for accurate identification. Most of the gastropods of the Palaeozoic Era belong to primitive groups, a few of which still survive. By the Carboniferous period many of the shapes seen in living gastropods can be matched in the fossil record.

It was mainly during the Mesozoic Era that the ancestors of many of the living gastropods evolved. In rocks of the Mesozoic Era, gastropods are also slightly more common as fossils, their shells are often well preserved. Their fossils occur in ancient beds deposited in both freshwater and marine environments.

Rocks of the Cenozoic Era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.