The Atlantic cod has two distinct color phases, gray-green and reddish brown. Its average weight is 10 to 25 lb (4.5-11.3 kg), but specimens weighing up to 200 lb (90 kg) have been recorded. Young Atlantic cod or haddock prepared in strips for cooking is called scrod. Cods feed on mollusks, crabs, starfish, worms, squid, and small fish. Some migrate south in winter to spawn. A large female lays up to five million eggs in midocean, a very small number of which survive. The Pacific cod is found N of Oregon. The tomcod resembles a young Atlantic cod with long, tapering ventral fins. It rarely exceeds 15 in. (37.5 cm) in length and lives close to shore. There is also a Pacific tomcod. The pollack, also called coalfish or green cod, is a plump olive-green cod found in cool waters of the Atlantic. Pollacks have forked tails and pale lateral lines and grow to 3 ft (90 cm) and 30 lb (13.6 kg).

The haddock is the most important food fish of Atlantic waters; most of the large annual catch is marketed frozen. It is also found in colder European waters. Haddocks are also bottom-feeders but are found in deeper water (up to 100 fathoms). They are smaller than cods, reaching 30 lb (13.6 kg) and a length of 3 ft (90 cm), and have black lateral lines and dark side patches. Finnan haddie is lightly smoked haddock. The burbot is the only freshwater cod, found deep in northern streams and lakes. It has a single barbel on its chin. A similar burbot is found in Europe and Asia. Lings and hakes, closely related to the cod, are fishes of commercial importance found in warmer waters. More slender than the cod, they are strong swimmers, preying on crustaceans and small fish.

Cods are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Gadiformes, family Gadidae.

Oil obtained primarily from the liver of the Atlantic cod and related fish. It is principally a mixture of the glycerides (see glycerol) of many fatty acids, but its minor constituents, the fat-soluble vitamin A and vitamin D, give it its importance. It was once used to treat and prevent rickets, but the widespread fortification of milk with vitamin D in the United States and Europe beginning in the 1930s eliminated rickets as a significant public health problem. It is still used as a remedy for joint pain caused by arthritis and as a preventive of cardiovascular disease, although these benefits have not been proven scientifically. It is also used in feeds for poultry and other animals.

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Large and economically important marine fish (Gadus morhua, family Gadidae) found on both sides of the North Atlantic, usually near the bottom in cold water. It ranges from inshore regions to deep waters. It is valued for its edible flesh, the oil of its liver, and other products. The cod is dark-spotted and ranges from greenish or grayish to brown or blackish; it may also be dull to bright red. It usually weighs up to about 25 lbs (11.5 kg) but can reach a maximum length and weight of more than 6 ft (1.8 m) and 200 lbs (91 kg). It feeds largely on other fishes and various invertebrates.

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Peninsula, eastern Massachusetts, U.S. Some 65 mi (105 km) long and 1–20 mi (2–32 km) wide, it touches Buzzards Bay and extends into the Atlantic Ocean in a wide curve, enclosing Cape Cod Bay. The Cape Cod Canal, cutting across the base of the peninsula, forms part of the Atlantic Intracoastal Waterway. Named by an English explorer who visited its shores in 1602 and took aboard a “great store of codfish,” Cape Cod was the site, near Provincetown, of the Pilgrims' landing in 1620. Extending into the warm Gulf Stream, it has coastal towns and villages that become densely populated resorts in summer. In the 19th century Provincetown was an active whaling port. The cape's northern hook was designated the Cape Cod National Seashore in 1961.

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