The Great Blue Heron , Ardea herodias, is a wading bird in the heron family Ardeidae, common over most of North and Central America as well as the West Indies and the Galápagos Islands, except for the far north and deserts and high mountains where there is no water for it to feed in. It is an extremely rare vagrant to Europe, with records from Spain, the Azores and England.

It is the largest North American heron, with a head-to-tail length of 91–140 cm (36-55 in), a wingspan of 167-201 cm (66-79 in), and a weight of 2–3.6 kg (4.4-8 lbs). It is blue-gray overall, with black flight feathers, red-brown thighs, and a paired red-brown and black stripe up the flanks; the neck is rusty-gray, with black and white streaking down the front; the head is paler, with a nearly white face, and a pair of black plumes running from just above the eye to the back of the head. The feathers on the lower neck are long and plume-like; it also has plumes on the lower back at the start of the breeding season. The bill is dull yellowish, becoming orange briefly at the start of the breeding season, and the lower legs gray, also becoming orangey at the start of the breeding season. Immature birds are duller in color, with a dull blackish-gray crown, and the flank pattern only weakly defined; they have no plumes, and the bill is dull gray-yellow.del Hoyo, J., Elliott, A., & Sargatal, J., eds. (1992). Handbook of the Birds of the World Vol. 1. Lynx Edicions, Barcelona ISBN 84-87334-10-5.Sibley, D. (2000). The Sibley Guide to Birds. National Audubon Society ISBN 0-679-45122-6Dickinson, M. B. et al., eds. (1999). Field Guide to the Birds of North America. National Geographic ISBN 0-7922-7451-2.

There are five subspecies:

• Ardea herodias herodias Linnaeus, 1758. Most of North America, except as below.
• Ardea herodias fannini Chapman, 1901. The Pacific Northwest from southern Alaska south to Washington; coastal.
• Ardea herodias wardi Ridgway, 1882. Kansas and Oklahoma to northern Florida.
• Ardea herodias occidentalis Audubon, 1835. Southern Florida, Caribbean islands.
• Ardea herodias cognata Bangs, 1903. Galápagos Islands.

The subspecies differ only slightly in size and plumage tone, with the exception of subspecies occidentalis, which as well as normal colored birds, also has a distinct white morph, known as the Great White Heron. This was long thought to be a separate species, and is mainly found near salt water. Birds intermediate between the normal morph and the white morph are known as Wurdemann's Heron; in these only the head is white.

The call is a harsh croak; they are most vocal during the breeding season, but will call occasionally at any time of the year in territorial disputes or if disturbed.