Regardless of their final habitat, all eels probably pass through the leptocephalus stage, an extended larval phase, in the open ocean and undergo metamorphosis to a juvenile stage that is a smaller version of the adult. At maturity, eels range from 10 cm (4 inches), in the deep sea Cyema atrum, to 3.5 metres (11.5 feet), in the moray Thyrsoidea macrura. Eels occur to considerable depths in most oceans and are greatly diverse in tropical seas. They range in colour from drab gray or black in deep-sea species to colourful and patterned in tropical reef species. Only the freshwater eels (family Anguillidae), which are in places abundant and greatly valued as food, are of major economic importance.

Eels have a remarkable life cycle. Broadly, it consists of development and early growth in the open ocean: the planktonic (free-floating) dispersal of eggs and larvae, metamorphosis, juvenile and adult growth, and the migration of maturing adults to an oceanic spawning area. Eels share the leptocephalus phase with several other orders (Elopiformes (elopiform) 【tarpons (tarpon) and relatives】, Saccopharyngiformes 【gulpers (gulper) and relatives】, and Albuliformes 【bonefishes (bonefish)】). A prolarva, hatching from a relatively large egg (up to 2.5 mm 【about 0.1 inch】 in diameter), rapidly becomes a leaflike leptocephalus, which floats in the surface layers of the open ocean for as long as two and a half years before metamorphosing.

Although the leptocephali were once thought to have been fishes (fish) of a distinct group, their relationship with the Anguilliformes was soon recognized from transitional specimens that showed larval and adult characters. They proved so difficult to identify, however, that new larval types were named as species of the genus Leptocephalus (though they cannot actually be considered different species from the adults that produced them), accounting for the several hundred forms known.

Leptocephali are not uncommon in the upper 500 metres (roughly 1,600 feet) of the ocean, a distribution that may be associated with the availability of food (diatoms and minute crustaceans). Their predators include various pelagic fishes. In tropical eels, larval life is possibly four to six months, but temperate species may spend upward of a year as larvae. During this time leptocephali, in the presence of suitable currents, may disperse widely from the adult spawning area. Working on massive collections of larvae from 1905 to 1930, a Danish biologist, Johannes Schmidt, established the early life history of the European and American freshwater eels. Although parts of his work have been questioned, his description of a western Atlantic spawning and a trans-Atlantic dispersal of leptocephali of these eels still stands.

After reaching full growth, the larva begins a rapid metamorphosis in which the body undergoes several progressive changes. The body becomes cylindrical and greatly reduced in bulk, perhaps by as much as 90 percent by weight, and the anal vent advances from its subterminal position to about the midpoint. The larval teeth are lost, the snout becomes rounded, the dorsal fin originates farther forward, and the larval melanophores (black pigment cells) disappear. Other changes, such as the loss of the pectoral fins or a reduction of body length, may also occur.

Leptocephali are markedly unlike their adults, and the metamorphic changes are so great that a fundamental problem arises in the correlation of the great variety of known leptocephali with their adults. Metamorphosis has been observed in aquariums and deduced in the oceans from progressive growth series in plankton samples. Certain characters survive metamorphosis and are important in the recognition of eel species. These include the number of muscle segments (myomeres); the development of dorsal, anal, and caudal fin rays; and the relative positions of the renal vessels and the gallbladder. In many leptocephali the larval melanophores also remain in the juvenile (or elver) stage.

Metamorphosis involves physiological and behavioral as well as structural changes, particularly those related to the assumption of a deep-sea, shallow-water, or freshwater mode of life. Metamorphosis is the mechanism by which the leptocephalus, after a period of growing, feeding, and competing with other similarly organized planktonic animals, can enter a markedly different habitat where body shape, differentiated feeding mechanisms, sense organs, and body coloration play an important role in survival. Metamorphosis in all eels is probably completed in the open ocean. The annual invasion of fresh waters by Anguilla elvers is a locally well-known process; it occurs during October–March in Europe and in spring in other temperate regions.

During several years' growth to maturity, eels are essentially carnivores, feeding diversely on planktonic or benthic (bottom-living) animals. Maturity is reached after about 10 years in the European freshwater eel (A. anguilla) but possibly much earlier in tropical marine species. The process of growth and maturation has been most closely studied in the European freshwater eel. In this species, both sexes pass through successive phases of neutrality, precocious feminization, and juvenile hermaphroditism prior to becoming definitively male or female, the sex being determined mainly by environmental factors.

All eels apparently undergo a short or long distance migration at maturity to a spawning area within the area of adult distribution (in most tropical marine eels) or some distance from it (in temperate Anguillidae and Congridae). These areas are generally located over the continental slope or in ocean basins some distance offshore.

During their juvenile and adult life, most eels are solitary fishes, swimming slowly by means of sinuous lateral movements of the body and median fins. Some species burrow rapidly, using a pointed tail and backward body movements. Morays (moray) and congers (conger eel) inhabit rock crevices, while certain congrids (Heterocongrinae, garden eels) form vast colonies of several hundred individuals in tropical reef areas.

An eel is distinguished externally from most other fishes (fish) by its elongated body, which is seldom laterally compressed. A continuous dorsal, anal, and caudal fin runs around the tail tip; pelvic fins are always absent; and gill openings are usually reduced. The body covering is usually scaleless. Minor departures from this overall body plan occur in the various eel families and are correlated well with different modes of life.

Typically, a leptocephalus is elongate, laterally compressed, transparent, and gelatinous, with prominent W-shaped myomeres and sharp forwardly directed larval teeth. At full growth, eel larvae are 5 to 10 cm (roughly 2 to 4 inches) long but may be much larger, 45 cm (about 18 inches) in the case of the snipe eel Nemichthys scolopaceus. Leptocephali are at least as diverse morphologically as their adults: some are filamentous, while others are deep-bodied, even resembling a small dinner plate in shape and size.

Pectoral and median fins are present in most leptocephali but may disappear during metamorphosis. Eyes are usually normal in shape but are occasionally telescopic, and the rostrum (an anterior beaklike projection) may be greatly extended forward from the snout. The attenuate viscera are located along the ventral aspect, below the myomeres. There is usually a straight unmodified intestine, and the anal vent is often forward of the tail. In some families the larval gut is swollen or festooned at various points along its length, a modification of unknown significance. A long liver, with a well-defined gallbladder, occurs anteriorly on either side of the intestine. The developing adult kidney lies at about mid-length. The organs are supplied and drained by many vertical blood vessels, of which the last one or two are the largest. The position of the last of these vessels equates well with the division between precaudal and caudal vertebrae and indicates the approximate position of the adult kidney.

In most leptocephali melanophores occur almost anywhere on or in the body. These vary from minute and compact to large and irregularly shaped, often distributed along the gut and laterally in a variety of patterns.

In adults there is a strong tendency toward a reduction or loss of fins and a streamlining of the profile, sometimes also with an attenuation of the body. The gill region is variously elongated by a posterior displacement of the gill arches, accompanied by separation of the pectoral girdle from the cranium. There are well-developed pharyngeal tooth plates, and the pharynx has assumed an important function in the movement of food into the esophagus. It also serves as an effective pump in the virtual absence of suction by the gill covers (opercula) for the passage of the respiratory current. The opercular series is much reduced, although numerous curved branchiostegal rays (internal gill supports) are strongly developed to support the long throat wall. Respiration through the skin is important in Anguilla and probably also in other eels.

Studies of the few known fossil eels and the comparative anatomy of adults and larvae suggest that eels arose in the Cretaceous Period (145.5–65.5 million years ago). Eels descended from two or more types that had at least some characters of the Elopiformes (tarpons (tarpon) and relatives) and Albuliformes (bonefishes (bonefish)).

The important characters in determining the taxonomic ranking of eels include the number and arrangement of myomeres and fin rays; the relative positions of mouth, nostrils, gill openings, renal vessels, and gallbladder; and the number and arrangement of teeth, skull bones, and vertebrae. Also important in identification are larval structures and pigmentation patterns.

The classification used in this article is a synthesis of the work of many authorities; however, it is based primarily on the work of Canadian biologist J.S. Nelson, who divides Anguilliformes into the suborders Anguilloidei, Muraenoidei, and Congroidei. It is expected that eel classification will be further modified as more is learned about the little-known families.

Initially the eels were split into the Colocephali (morays) and Enchelycephali (others), and the suborder Carenchelyi for the Derichthyidae was added later. The current classification recognizes three suborders, the Anguilloidei, Muraenoidei, and Congroidei, although the higher-level classification of the order Anguilliformes is still in dispute. A review of eel phylogeny by British paleontologist Peter L. Forey and colleagues has suggested the inclusion of the saccopharyngoid fishes—that is, the gulper eels (order Saccopharyngiformes)—in the orderAnguilliformes. Overall detailed species-level work has resulted in a good understanding of generic and species limits, although much remains to be discovered about eel taxonomy.