The Great Northern Diver, known in North America as the Common Loon (Gavia immer [GAY-vee-ah IM-mer]), is a large member of the loon, or diver, family. Adults can range from 61-100 cm (24-40 inches) in length with a 122-152 cm (4-5-foot) wingspan, slightly smaller than the similar White-billed Diver or "Yellow-billed Loon". The weight can vary from 1.6 to 8 kg (3.6 to 17.6 lbs). On average a Common Loon is about 81 cm (32 inches) long, has a wingspan of 136 cm (54 inches), and weighs about 4.1 kg (9 lbs).

The Great Northern Diver breeds in Canada, parts of the northern United States, Greenland, and Alaska. There is a smaller population (ca. 3000 pairs) in Iceland. On isolated occasions they have bred in the far north of Scotland. The female lays 1 to 3 eggs on a hollowed-out mound of dirt and vegetation very close to water. Both parents build the nest, sit on the egg or eggs, and feed the young.

This species winters on sea coasts or on large lakes over a much wider range in Northern Europe and the British Isles as well as in North America.

Breeding adults have a black head, white underparts, and a checkered black-and-white mantle. Non-breeding plumage is brownish, with the chin and foreneck white. The bill is grey or whitish and held horizontally. The bill colour and angle distinguish this species from the similar White-billed Diver.

This species, like all divers, is a specialist fish-eater, catching its prey underwater, diving as deep as 200 feet (60 m). Freshwater diets consist of pike, perch, sunfish, trout and bass; salt water diets consist of rock fish, flounders, sea trout and herring. The bird needs a long distance to gain momentum for take-off, is ungainly on landing. It is clumsy on the land due to the position of the legs at the rear of the body, ideal for diving but not well-suited for walking. Birds land on water skimming along on their bellies to slow down rather than their feet, as these are set too far back. It swims gracefully on the surface, dives as well as any flying bird, and flies competently for hundreds of miles in migration. It flies with its neck outstretched, usually calling a particular tremolo that can be used to identify a flying loon.

These birds have disappeared from some lakes in eastern North America due to the effects of acid rain and pollution, as well as lead poisoning from fishing sinkers and mercury contamination from industrial waste. Artificial floating nesting platforms have been provided for loons in some lakes to reduce the impact of changing water levels due to dams and other human activities.

The Great Northern Diver is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies.

This diver is well-known in Canada, appearing on the "loonie" coin and the previous series of $20 bill, and is the provincial bird of Ontario. Also, it is the state bird of Minnesota.

The voice and appearance of the Common Loon has made it prominent in several Native American tales. These include a story of a loon which created the world in a Chippewa story; a Micmac saga describes Kwee-moo, the loon who was a special messenger of Glooscap, the tribal hero; native tribes of British Columbia believed that an excess of calls from this bird predicted rain, and even brought it; and the tale of the loon’s necklace was handed down in many versions among Pacific Coast peoples. Folk names include big loon, black-billed loon, call-up-a-storm, ember-goose, greenhead, guinea duck, imber diver, ring-necked loon, and walloon.

Gavia is Latin for "sea smew" (although divers are not Smew). The specific meaning of immer either is:

related to Swedish immer and emmer, the grey or blackened ashes of a fire, referring to its dark plumage; or

Latin immergo, to immerse, and immersus, submerged.