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The Cephalopods (Greek meaning "head-foot") are an important class of molluscs. They have bilateral symmetry, a head, and arms or tentacles.Teuthology, a branch of malacology, is the study of cephalopods.

The class has two living subclasses. In the Coleoidea, the mollusc shell has become smaller, or is not there at all; this subclass has the octopus, squid, and cuttlefish. The Nautiloidea have a shell; Nautilus is its only living genus.

There are at least 800 different living species of Cephalopods. Two important extinct taxa are the Ammonites, and the Belemnites (order Belemnoidea, of class Coleoidea). Cephalopods are found in all the oceans of the world and at all pelagic levels. None of them can live in freshwater (water with no salt in it), but a few species live in brackish (partly salty) water.