any member of the order Pleuronectiformes, which includes about 600 species of bony, oval-shaped, flattened fishes such as the flounder, halibut, and turbot. The pleuronectiforms are unique among fishes in being asymmetrical. They are strongly compressed, with both eyes on one side, while other fishes and vertebrates in general are bilaterally symmetrical. The asymmetry is believed to have evolved from a generalized, symmetrical percoid (sea bass) body pattern in a fish that habitually rested on its side. Larval flatfishes have an eye on each side of the head, but during a period of rapid body change (metamorphosis) one eye migrates to the other side of the head, after which the larvae settle to the bottom. Osteological changes resulting from the eye migration are responsible for the asymmetry in the flatfish skull.

Flatfishes of the family Pleuronectidae are commercially important (commercial fishing) in northern waters, and members of other families are taken in limited quantities. Some Bothidae and Soleidae (soles (sole)) are exploited in tropical and temperate waters, but no other flatfishes are utilized to the extent that Pleuronectidae are.

Flatfishes are primarily found in temperate and tropical seas, with some species extending northward into the Arctic. Sizes range from about 100 millimetres (four inches) to the large Atlantic halibut, which attains a length of more than two metres (nearly seven feet) and a weight of about 325 kilograms (716 pounds). Most species are marine, but some spend all or part of their life in freshwater. Flatfishes are found in depths up to 1,000 metres (3,300 feet), but most occur on the continental shelf in less than 200 metres of water.

Flatfishes generally spawn offshore, but some spawn in estuaries. Fecundity is high, females generally releasing at least several hundred thousand eggs (large female halibut have between 2,500,000 and 3,000,000 eggs). The eggs are small and float freely (pelagic) or sink to the botton (demersal), with or without oil globules. Newly hatched larvae are 1.5 to three millimetres long (approximately ¹/₁₆ to ¹/₈ inch). Active feeding begins shortly after hatching, and mortality of newly hatched larvae is extremely high. Larvae drift with currents (planktonic) until metamorphosis, or shortly afterward, and then settle to the bottom to assume their adult bottom-living (benthic (benthos)) existence. Swimming is accomplished by undulating movements of body and fins, rather than by sculling with the caudal fin as in most other fish.

Flatfishes lie on the bottom, generally covered by sand or mud, with only their eyes protruding. The eyes can be raised or lowered and moved independently. Flounders feed primarily on crustaceans, other bottom invertebrates, and small fish. When feeding they remain motionless until their prey ventures too close and then literally leap off the bottom in pursuit. Flatfishes in turn fall prey to a variety of large fish and cetaceans (whales, porpoises, etc.), but the primary predator of flatfishes is man.

Many species display sexual dimorphism, with the male having one or several of the following characteristics: elongated pectoral-fin rays, wider interorbital bones, spines on the head, tentacles on the eyes, more elaborate pigmentation. Flounders have a long dorsal fin extending from the head to caudal (tail) fin and an anal fin extending from vent (anus) to caudal fin in most species. Pectoral fins are present on all larval flatfishes but are lost or reduced in adults of the families Soleidae and Cynoglossidae. Caudal-fin rays and their supporting structures are variable. Scales are ctenoid (rough edged) or cycloid (smooth). Dentition is variable and corresponds to feeding habits of the species. Active predators have large, well-developed teeth in both jaws, whereas those living primarily in mud and feeding on bottom invertebrates have teeth only on the lower jaw of the blind side. Sexes are easily distinguished because the ovaries extend posteriorly from the body cavity beneath the skin and a thin layer of muscle immediately above the muscles of the anal-fin rays. Testes cannot be seen without dissection. The stomach and intestines curl within the body cavity to form a loop.

The main feature of metamorphosis is the migration of the eye around or through the head. This is accomplished as a movement either over the middorsal ridge or through the head, in a depression between the supraorbital bars (over the eye) and ventral edge of the dorsal fin. The supraorbital bars extend forward from the cranium to the ethmoid region of the skull (the area in front of the eye), gradually shifting ventrally and coming to lie next to one another. As the eye migration begins, the dorsal edge of the supraorbital bar is reabsorbed to make room for the eye moving through the head. The supraorbital bars ossify and become the interorbital bone after the eye has completed its migration. The blind- (bottom-) side frontal bone shifts to the ocular (upper) side and forms a portion of the optic-capsule floor for the upper eye. Torsion (twisting) of the frontals, ethmoid, and mouthparts is the essential feature of the flatfish skull.

Normal pigmentation on adult flatfishes consists of a coloured ocular side and an unpigmented (white) blind side. The ocular side is variable in pigment pattern and intensity. Flatfishes can mimic their background by assuming a similar coloration. Partial or complete albinism is known in some species, but a more common colour variation is ambicoloration (coloration on both sides). Ambicoloration can be partial or complete and is often associated with incomplete migration of the eye (in which the migrating eye stops on middorsal ridge) and a hooked appearance, caused by the unattached origin of the dorsal fin. Reversal (eyes and pigmentation on the side that normally is unpigmented) is fairly frequent in some species but quite rare in others.

Flatfishes are divisible into three suborders and seven families.