The two valves or a bivalve shell cover the right and left sides of the animal; they are hinged dorsally (above the body) and open ventrally (below the body). Usually the two valves are similar and equal in size, but in some forms, such as the oyster, that attach to the substratum by one valve (i.e., lying on their sides), the left-hand (or upper) valve is larger than the right-hand (or lower) one. Two muscles, called adductors, run between the inner surfaces of the two valves; acting antagonistically to the hinge ligament, they enable the shell to close rapidly and tightly.

Because of the enormous variety of sizes, shapes, surface sculpturing, and colors, shell characteristics are of great importance in the identification and classification of bivalves. Shells range in size from the tiny (1/16-in./2-mm) seed shells characteristic of members of the freshwater family Sphaeriidae to the giant clam, Tridacna, of the South Pacific, which attains a length of over 4 ft (120 cm) and may weigh over 500 lb (225 kg).

Within the shell is a fleshy layer of tissue called the mantle; there is a cavity (the mantle cavity) between the mantle and the body wall proper. The mantle secretes the layers of the shell, including the inner nacreous, or pearly, layer. Sometimes a pearl is formed as a reaction to irritation, by the depositing of nacreous layers around a foreign particle. The head is much reduced, without eyes or tentacles, and a muscular hatchet-shaped foot projects from the front end of the animal, between the valves. The foot is used for burrowing, and, in some bivalves (e.g., razor clams), to swim. Many bivalves have two tubes, or siphons, extending from the rear end: one (the incurrent siphon) for the intake of oxygenated water and food and one (the excurrent siphon) for the outflow of waste products. The two tubes may be joined in a single siphon, or "neck."

The gills, suspended within a mantle cavity, are usually very large and function in food gathering (filter feeding) as well as in respiration. As water passes over the gills, tiny organic particles are strained out and are carried to the mouth. Members of the order Septibranchia, however, lack gills and feed on small crustaceans and worms.

Bivalves have a complete digestive tract; a reduced nervous system; a complete, open circulatory system with a chambered heart, arteries, veins, and blood sinuses; and excretory and reproductive organs. In most species the sexes are separate, and the eggs and sperm are shed into the water, where fertilization occurs. The larval stage is free-swimming and lacks a shell.

Bivalves differ in their habits: some, such as the oysters and marine mussels, have a reduced foot and are permanently attached to a substratum; some, such as the clams and freshwater mussels, burrow slowly through the sand or mud using the foot; some, such as the cockle shells, live on or near the surface of the ocean floor; still others, such as the shipworm, burrow through rocks or wood seeking protected dwellings and do damage to rock pilings and other marine installations. The scallops swim with great speed by suddenly clapping the shell valves together and ejecting water from the mantle cavity. Bivalves that are exposed at low tide, such as the marine mussels, keep their gills wet with water retained in the mantle cavity.

Bivalves are classified in the phylum Mollusca, class Pelecypoda or Bivalvia.