Eels are snakelike fish with elongated bodies and shallow fins. The eel can be found in both fresh and saltwater, depending upon the species, although all eels breed in salt water. Many species of freshwater eel are consumed by humans, and eel is popular in European and Asian cuisines. Some species of eel harvested by humans are under threat, and several nations have undertaken measures to ensure the health of their eel stocks.

The eel is a very peculiar fish, with an imperfectly understood life cycle. All eels spawn at sea, many of them in the Sargasso Sea. The eel larva drift with the currents as they mature into young eels called glass eels, a totally transparent young form of the eel. Glass eels are often found coastal areas, and in the autumn the glass eels move into estuaries, where they turn into pigmented young eels known as elvers. Elvers migrate upriver, often over very long distances, and many species spend their entire lives in fresh water. In this stage, the eel is known as a yellow eel, and will reach a mature length which can range between two and five feet (.5-1.5 meters) in length, depending upon the species.

When the eels reach sexual maturity, they undergo additional physical changes, turning gray with a pale belly. The pectoral fins and eyes of the eel enlarge, presumably to assist the fish in its migration back to the spawning area at sea, where the eel will die after participating in spawning activity. Since scientists have not actually witnessed the eel spawning process, biological information about the early life of eels is primarily conjecture.

Some eels such as the moray spend their entire lives at sea, lurking in crevasses and deeper ocean water. The moray is a well known family, because morays tend to be vicious and are equipped with extremely sharp teeth which have been known to chip bone. Swimmers are encouraged to avoid the moray eel, although it can be perfectly harmless and even friendly if it does not feel threatened. Many fish have the common name of eel, such as the electric eel, although they are not in fact true eels.

Freshwater eels are commercially fished by several nations, and the global stocks of the fish are in decline. They may partially be due to the fact that the fish can take up to 20 years to mature, and some countries may have fished out their stocks of potentially viable mature fish. Because the life of the eel is not fully understood, the damage caused by commercial fishing may be more severe than was originally thought. Several nations are working together to conserve and rebuild their eel stocks, since the eel provides vital economic and cultural benefits to many countries.