Shell or shell, also called shell for short, is a hard protective outer layer created by animals living in the sea. A shell is a part of an animal's body. Empty shells are often washed on the beach by vagrants. The shell is empty because the animal is dead and the soft part is eaten or rotted by another animal.

The term shell usually refers to the exoskeleton of an invertebrate (an animal without a skeleton), usually composed of calcium carbonate or chitin. Most shells found on the beach are shells of marine mollusks, in part because they are usually made of calcium carbonate and have better durability than crustaceans.

In addition to mollusc shells, other shells found on the beach are barnacles, horse crabs and brachiopods. The marine nematode worms of the Serpulidae family produce shells, which are tubes made of calcium carbonate and stick to other surfaces. The shells of sea urchins are called tests, while the molts of crabs and lobsters are called exuviaes. Although most shells are outer shells, some cephalopods have inner shells.

Shells have been used for many different purposes throughout history and prehistory. However, shells are not the only shells. In various habitats, there are shells of freshwater animals (such as freshwater mussels and freshwater snails) and shells of snails.