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 in length 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.

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 [tarpons and relatives], Saccopharyngiformes [gulpers and relatives], and Albuliformes [bonefishes]).