Seals, Sea lions and Walrus are currently placed in a Sub-order, the Pinnipedia, of the Order Carnivora which also includes the bears, dogs, racoons, weasles (including otters), hyaenas cats, and mongooses. There have many recent studies on the genetic and fossil history of these groups and their place in the scheme of mammalian classifcation may well change in the near future.

Three families of living pinnipeds are recognized, the Phocidae (hair seals or true seals), the Otaridae (fur seals and sea lions) and the Odobenidae (walrus). The term pinnipedia translates from Latin as "fin foot". All of these animals must come ashore to breed, give birth and nurse their young, though some species are at sea for several months at a time while others return to the shore every day.

There are currently four species of pinnipeds in the U.S. on the endangered species list. These include the Caribbean monk seal, the Guadalupe fur seal, the Hawaiian monk seal and the Steller sea lion. The Caribbean monk seal and the Hawaiian mink seal are listed as endangered, while the Guadalupe fur seal and Steller sea lion are currently listed as threatened.

Commercial hunting of seals in the 18th and 19th century, and in the early years of this century played a large role in pinniped population declines. Other factors involved have been coastal development and competition with man for prey species.

The Hawaiian monk seal was listed as endangered throughout its range on November 23, 1976. Counts have been made at the atolls, islands and reefs where they haul out in the northwest Hawaiian Islands since the late 1950s. By 1982, the population had declined to half of its 1957-1958 level. Since the mid-1980's, beach counts have declined at five percent per year. NMFS estimates that currently there are approximately 1400 animals. The number of births declined significantly at all five major breeding locations in 1990, followed by some recovery in subsequent years. However, the number of births has not reached the level observed in the mid-to-late 1980's, and is not expected to in the near future because of the high losses of immature seals at French Frigate Shoals and mobbed seals at Laysan and Lisianski Islands.

The Caribbean monk seal was listed as endangered throughout its range on April 10, 1979. The last reliable sighting of a Caribbean monk seal occurred in 1952. None were seen in aerial surveys in 1973, and no confirmed sightings have been reported since then. Many scientists believe that the species has been extinct since the early 1950s. No recovery effort is being made for this species.

The Steller or northern sea lion was listed as threatened throughout its range on December 4, 1990. Steller sea lion population west of 144 degrees wes t longitude was declared endangered as of June 1997. The centers of abundance and distribution are the Gulf of Alaska and Aleutian Islands, respectively. Rookeries (breeding colonies) are found from the central Kuril Islands to Ano Nuevo Island, California; most large rookeries are in the Gulf of Alaska and Aleutian Islands. More than 50 Steller sea lion rookeries and a greater number of haulout sites have been identified. Species abundance estimates during the late 1970s ranged from 248,000 to 300,000 adult and juvenile animals. However, counts at rookeries and haulout sites throughout most of Alaska and the USSR in 1989, plus estimates from surveys conducted in recent years at locations not counted in 1989, provide a range-wide Steller sea lion population estimate of about 116,000. In 1994, an estimated 67,100 occurred in U.S. waters.