Bivalves are molluscs belonging to the class Bivalvia. They have two-part shells, and typically both valves are symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.

Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves. Bivalves feed by siphoning and filtering large particles from water. Some bivalves attach themselves to surfaces in the water, while others bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as scallops, can swim.