The Great Auk (Pinguinus impennis), formerly of the genus Alca, was a large, flightless alcid that became extinct in the mid-19th century. It was the only modern species in the genus Pinguinus, a group of birds that formerly included one other species of flightless giant auk from the Atlantic Ocean region. It bred on rocky, isolated islands with easy access to both the ocean and a plentiful food supply, a rarity in nature that provided only a few breeding sites for the auks. When not breeding, the auks spent their time foraging in the waters of the North Atlantic, ranging as far south as New England and northern Spain through Canada, Greenland, Iceland, the Faroe Islands, Norway, Ireland, and Great Britain.

The Great Auk was 75 to 85 centimetres (30 to 33 in) tall and weighed around 5 kilograms (11 lb), making it the largest member of the alcid family. It had a black back and a white belly. The black beak was heavy and hooked with grooves on its surface. During summer, the Great Auk had a white patch over each eye. During winter, the auk lost this patch, instead developing a white band stretching between the eyes. The wings were 15 centimetres (5.9 in) long, rendering the bird flightless. Instead, the auk was a powerful swimmer, a trait that it used in hunting. Its favorite prey were fish, including Atlantic Menhaden and Capelin, and crustaceans. Although agile in the water, it was clumsy on land. Its main predators were Orcas, White-tailed Eagles, Polar Bears, and humans. Great Auk pairs mated for life. They nested in extremely dense and social colonies, laying one egg on bare rock. The egg was white with variable brown streaking. Both parents incubated for about six weeks before their young hatched. The young auks left the nest site after two or three weeks and the parents continued to care for them.

Great may refer to:

An auk is a bird of the family Alcidae in the order Charadriiformes. Auks are superficially similar to penguins due to their black-and-white colours, their upright posture and some of their habits. Nevertheless they are not closely related to penguins, but rather are believed to be an example of moderate convergent evolution.

Unlike penguins, modern auks can fly (except for the recently extinct Great Auk). They are good swimmers and divers, but their walking appears clumsy. Due to their short wings, auks have to flap their wings very quickly in order to fly. Extant auks range in size from the Least Auklet, at 85 g (3 oz) and 15 cm (6 in), to the Thick-billed Murre, at 1 kg (2¼ lb) and 45 cm (18 in).

Auks live on the open sea and only go ashore for breeding, although some species, like the Common Guillemot, spend a great part of the year defending their nesting spot from others.

Several species have different names in Europe and North America. The guillemots of Europe are murres in North America, if they occur in both continents, and the Little Auk becomes the Dovekie.