The bald eagle is actually a sea eagle (Haliaeetus species) that commonly occurs inland along rivers and large lakes. The adult male is about 90 cm (36 inches) long and has a wingspan of 2 metres (6.6 feet). Females, which grow somewhat larger than males, may reach 108 cm (43 inches) in length and have a wingspan of 2.5 metres (8 feet). Both sexes are dark brown, with a white head and tail. The bird is not actually bald; its name derives from the conspicuous appearance of its white-feathered head. The beak, eyes, and feet are yellow.

The bald eagle's nest is a large platform of sticks built atop a large, isolated tree or pinnacle of rock located within easy flight of water. Nests are usually about 1.5 metres (5 feet) wide, but old nests can be almost twice this size. The two or three eggs laid within take slightly longer than a month to hatch. Both parents share in the incubation and feeding of the young. The immature birds are brown with whitish tail and wing linings, but the pure white head and tail plumage do not appear until the birds are four to five years old.

Bald eagles may have numbered in the hundreds of thousands when they were declared the American national bird in 1782, but their numbers steadily declined over the next two centuries owing to human activities and persecution. The birds were hunted for sport, for bounties offered by state and federal governments, and because they were thought to menace livestock. In Alaska, where eagles perched on fish traps and scared away the salmon (an annoyance eventually overcome by fitting the traps with devices to discourage perching), Alaskan bounty hunters killed more than 100,000 eagles in the period 1917–52. The U.S. government's Bald Eagle Protection Act of 1940 made it illegal to kill bald eagles (Alaska was exempt), but the birds' numbers continued to decline, primarily because of the effects of the pesticide DDT, which came into widespread agricultural use after World War II. This pesticide accumulated in the birds' tissues and interfered with the formation of the shells of their eggs; the thin, weak shells laid by heavily contaminated birds were easily broken and fewer young were produced. By the early 1960s, the number of bald eagles in the coterminous United States had dropped to fewer than 450 nesting pairs.

In 1972 the use of DDT was banned in the United States, and in 1978 the U.S. government declared the bald eagle an endangered species in all but a few of the northernmost states. By the late 1980s, these measures had enabled the birds to replenish their numbers in the wild. The bald eagle was reclassified from endangered to threatened status in 1995, by which time there were an estimated 4,500 nesting pairs in the lower 48 states. By 2000 the population had increased to more than 6,300 pairs, and the species was being considered for removal from the U.S. list of endangered species.

Like all hawks and eagles, the bald eagle belongs to the family Accipitridae of the order Falconiformes (falconiform).