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, or in deserts and high mountains where there is no water for it to feed in.

It is the largest North American heron, with a head-to-tail length of 91–137 cm (36-54 in), a wingspan of 180 cm (71 in), and a weight of 2.2–3.6 kg (4.8-8 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.

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 a 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.

It is found throughout most of North America, including Alaska, British Columbia, Quebec and Nova Scotia. The range extends south through Florida, Mexico and the Caribbean to South America. Great blue herons can be found in a range of habitats, in fresh and saltwater marshes, mangrove swamps, flooded meadows, lake edges, or shorelines, but they always live near bodies of water. Generally, they nest in trees or bushes that stand near a body of water. In general, what is shared in common is that all must be near or on a site of water for a living area (nest.)

It feeds in shallow water or at the water's edge during both the night and the day, but especially around dawn and dusk. Herons locate their food by sight and generally swallow it whole. Herons have been known to choke on prey that is too large. It uses its long legs to wade through shallow water, and spears fish or frogs with its long, sharp bill. Its diet can also include insects, snakes, turtles, rodents and small birds.

It is generally a solitary feeder. Individuals usually forage while standing in water, but will also forage in fields or drop from the air, or a perch, into water. As large wading birds, Great Blue Herons are able to feed in deeper waters, and thus are able to exploit a niche not open to most other heron species.

This species usually breeds in monospecific colonies, in trees close to lakes or other wetlands; often with other species of herons. These groups are called heronry (more accurately than "rookery"). The size of these colonies may be large, ranging between 5–500 nests per colony, with an average of approximately 160 nests per colony.

Great Blues build a bulky stick nest, and the female lays three to six pale blue eggs. One brood is raised each year. If the nest is abandoned or destroyed, the female may lay a replacement clutch. Reproduction is negatively affected by human disturbance, particularly during the beginning of nesting. Repeated human intrusion into nesting areas often results in nest failure, with abandonment of eggs or chicks.

Both parents feed the young at the nest by regurgitating food. Parent birds have been shown to consume up to 4 times as much food when they are feeding young chicks than when laying or incubating eggs.

Eggs are incubated for approximately 28 days and hatch asynchronously over a period of several days. The first chick to hatch usually becomes more experienced in food handling and aggressive interactions with siblings, and so often grows more quickly than the other chicks.

Birds east of the Rockies in the northern part of their range are migratory and winter in Central America or northern South America. From the southern United States southwards, and on the Pacific coast, they are year-round residents.

It has been recorded as a vagrant in Greenland, Hawaii, and the Azores.