The brown bear (Ursus arctos) is a species of bear distributed throughout the Northern hemisphere. Weighing up to 130–700 kg (290-1,500 pounds), the larger races of brown bear tie with the Polar bear as the largest extant land carnivores. The grizzly bear (Ursus arctos horribilis), the Kodiak Bear (Ursus arctos middendorffi), and the Mexican Grizzly are North American subspecies of the brown bear. However, DNA analysis has recently revealed that the identified subspecies of brown bears, both Eurasian and North American, are genetically quite homogeneous, and that their genetic phylogeography does not correspond to their traditional taxonomy. It is sometimes referred to poetically as the bruin, from Middle English, based on the name of the bear in History of Reynard the Fox, translated by William Caxton, from Middle Dutch bruun or bruyn.

Brown bears have furry coats in shades of blonde, brown, black, or a combination of those colors. The longer outer guard hairs of the brown bear are often tipped with white or silver, giving a "grizzled" appearance. They have a very short, stubby tail, just like all bears in the world. Brown bears have a large hump of muscle over their shoulders, which give strength to the forelimbs for digging. Bears are very powerful, even if considered pound for pound; a large specimen can break a neck or spine of a fully grown buffalo with a single blow. Forearms end in massive paws with very powerful claws up to 15 cm (5.9 inches) in length. Their heads are large and round with a concave facial profile. The normal range of physical dimensions for a brown bear is a head-and-body length of 1.7 to 2.8 m (5.6 to 9.2 feet) and a shoulder height 90 to 150 cm (35 to 59 inches), although the abnormally large specimens exceed these measurements. The smallest subspecies is the European brown bear, with mature females weighing as little as 90 kg (200 lb). The largest subspecies of the brown bear are the Kodiak bear and the bears from coastal Russia and Alaska. It is not unusual for large male Kodiak Bears to stand over 3 m (10 feet) while on their hind legs and to weigh about 680 kg (1,500 lb). Bears raised in zoos are often heavier than wild bears because of regular (sometimes excessive) feeding and limited movement. In zoos, bears may weigh up to 900 kilograms (2,000 pounds), like the well-known "Goliath" from New Jersey's Space Farms Zoo and Museum. According to the Great Bear Almanac[citation needed], one of the well known books about bears, the largest Kodiak bear weighed over 1,100 kilograms (2,500 pounds).

Claws are mainly used for digging. Unlike the claws of cats such as lions or tigers, brown bear claws are not retractable, giving them relatively blunt points.

In spite of their size, some have been clocked at speeds in excess of 56 km/h (35 mph). Along with their strength and deceptive speed, brown bears are legendary for their stamina. They are capable of running at full speed for miles at a time without stopping.

Brown bears were once native to Asia, the Atlas Mountains in Africa, Europe and North America, but are now extinct in some areas and their populations have greatly decreased in other areas. They prefer semi-open country, usually in mountainous areas.

Brown bears live in Alaska, east through the Yukon and Northwest Territories, south through British Columbia and through the western half of Alberta. Isolated populations exist in northwestern Washington, northern Idaho, western Montana, and northwestern Wyoming. Brown bears are also found in Minnesota, Wisconsin and Idaho.

The population of brown bears in the Pyrenees mountain range between France and Spain is so low, estimated at fourteen to eighteen with a shortage of females, that bears, mostly female, from Slovenia were released in the spring of 2006 to alleviate the imbalance and preserve the species' presence in the area, despite protests from French farmers.

There are about 200,000 brown bears in the world. The largest populations are in Russia, with 120,000, the United States, with 32,500, and Canada with 21,750. 95% of the brown bear population in the United States is in Alaska, though in the West they are repopulating slowly but steadily along the Rockies and plains. Much smaller populations, ranging in the hundreds, are also present in countries such as Mexico. In Europe, there are 14,000 brown bears in ten separate fragmented populations, from Spain to Russia and from Scandinavia in the north to Romania and Bulgaria in the south. They are extinct in the British Isles, extremely threatened or extinct in France, and in trouble over most of Central Europe. The brown bear is Finland's national animal. The Carpathian brown bear population is the largest in Europe outside Russia, estimated at 4,500 to 5,000 bears.

In Arctic areas, the potential habitat of the brown bear is increasing. The warming of that region has allowed the species to move farther and farther north into what was once exclusively the domain of the polar bear. In non-Arctic areas, habitat loss is blamed as the leading cause of endangerment, followed by hunting.

The brown bear has existed in North America since at least the most recent ice age, though it is thought that the larger, taller, and stronger giant short-faced bear or bulldog bear was the dominant carnivore at the time. The giant short-faced bear was a tall, thin animal adapted to eat large mammals, whereas the grizzly or brown bear has teeth appropriate for its omnivorous diet. The giant short-faced bear, on average, weighed twice as much as the grizzly, despite some exceptional grizzly bears in the later Old West that weighed 800 kilograms.

The brown bear also shared North America with the American lion and Smilodon, carnivorous competitors. The modern grizzly can eat plants, insects, carrion, and small and large animals. The American lion, Smilodon, and giant short-faced bear had a more limited range of food, making them vulnerable to starvation as the supply of available large mammals decreased, possibly due to hunting by humans.

The time of the Arctodus extinction is about the same as that of the long-horned Bison and other megafauna. Both of these animals were replaced by Eurasian immigrants, specifically the Brown Bear and American Bison. Since this was also about the same time as the Clovis tool kit hunting culture appeared in North America, with culturally advanced humans entering the Americas from Asia, the implication is that the brown bear was better adapted to human competition than the megafauna, presumably due to a long term coexistence in the Old World with people.

The extinction of ice-age herbivorous megafauna resulted in the extinction of the sabertooth, American lion, and giant short-faced bear, leaving the brown bear as the major large predator in North America, with the gray wolf, the jaguar in the south, the American black bear, and cougar also competing for large prey. The origin of human presence in America is widely accepted to have occurred across the Bering Land Bridge with the largest known immigration being that of the Paleo Indians at about the last ice age, bringing with them the Clovis point and advanced hunting techniques (see: Migration to the New World). When the last ice age ended about 10,000 years ago, brown bears from farther south in North America slowly expanded their range northward and back up into Alaska. Today there are three genetically distinct grizzly bear clades in North America: the Alaskan-Yukon Grizzly, the Alberta-Saskatchewan lineage, and those found in the Washington-Idaho-Montana-Wyoming area.

In Europe, the brown bear shared its habitat with other predators such as the Cave lion, Cave hyena and the larger, closely related Cave bear, which the brown bear ultimately outlasted. The cave bear was hunted by Neanderthals who may have had a religion relating to this bear, the Cave Bear Cult, but the Neanderthal population was too small for their consumption of cave bear to result in the species' extinction, and the cave bear outlasted the Neanderthals by 18,000 years, becoming extinct about 10,000 years ago. The cave bear and brown bear diets were similar, and the two species probably lived in the same area at the same time. Why the cave bear died out is not known.

The brown bear is primarily nocturnal and, in the summer, puts on up to 180 kg (400 pounds) of fat, on which it relies to make it through winter, when it becomes very lethargic. Although they are not full hibernators, and can be woken easily, both sexes like to den in a protected spot such as a cave, crevice, or hollow log during the winter months.

Normally a solitary animal, the brown bear congregates alongside streams and rivers during the salmon spawn in the fall. In alternate years, females produce one to four young, which weigh only about 1 to 2 kg (2 to 5 lb) at birth. Raised entirely by their mother, cubs climb trees when in danger.

Brown bears retrace their own tracks and walk only on rocks while being hunted to avoid being traced. Adults bears are generally immune from predatory attacks from anything other than another bear. However the Siberian tiger will prey on smaller-sized bears, and have attacked larger ones on some occasions.

They are omnivores and feed on a variety of plant parts, including berries, roots, and sprouts, fungi, fish, insects, and small mammals, especially ground squirrels. Despite their reputation, most brown bears are not particularly carnivorous as they derive up to 90% of their dietary food energy from vegetable matter. Their jaw structure has evolved to fit their dietary habits and it is longer and lacks strong, sharp canine teeth of true predators. Their diet varies enormously throughout their differing ranges. For example, bears in Yellowstone eat an enormous number of moths during the summer, sometimes as many as 40,000 in a day, and may derive up to a third of their food energy from these insects. Locally, in areas of Russia and Alaska, brown bears feed mostly on spawning salmon, and the nutrition and abundance of this food accounts for the enormous size of the bears from these areas. Brown bears also occasionally prey on deer (Odocoeilus spp.; Dama spp., Capreolus spp.), Red Deer (Cervus elaphus or American elk), moose (Alces alces) and Bison (Bison bison spp., Bison bonasus). When brown bears attack these animals, they tend to choose the young ones since they are much easier to catch. When hunting, the brown bear uses its sharp canine teeth for neck-biting its prey. They also feed on carrion and will drive wolves, cougars, black bears and Siberian tigers from their kills.

Like all bears the brown bear is plantigrade, meaning it walks with its entire foot like humans and weasels, rather than on its toes like cats and dogs, which are digitigrade. They can stand up on their hind legs for extended periods of time. Bears tend to sit down on their rear with their upper body off the ground.

Bears become attracted to human created food sources such as garbage dumps, litter bins, and dumpsters; and venture into human dwellings or barns in search of food as humans encroach into bear habitat. In the U.S., bears sometimes kill and eat farm animals. When bears come to associate human activity with a "food reward", a bear is likely to continue to become emboldened and the likeliness of human-bear encounters increases. The saying, "a fed bear is a dead bear," has come into use to popularize the idea that allowing bears to scavenge human garbage, pet food, or other food sources that draw the bear into contact with humans can result in a bear's death.

Relocation has been used to separate the bear from the human environment, but it does not address the problem bear's newly learned humans-as-food-source behavior. Nor does it address the environmental situations which created the human habituated bear. "Placing a bear in habitat used by other bears may lead to competition and social conflict, and result in the injury or death of the less dominant bear."

Some bears become hooked on a given food source and will return to the same location despite relocation. Bears that repeatedly return to a human environment for food are sometimes killed to prevent human injury. The phrase "a fed bear is a dead bear" has been used to describe this practice, and to encourage humans to prevent bears from obtaining food from human sources, such as trash cans and campers' backpacks.

Yellowstone National Park, an enormous reserve located in the Western United States, contains prime habitat for the Grizzly Bear (Ursus arctos horribilis), but due to the enormous number of visitors, human-bear encounters are common. The scenic beauty of the area has led to an influx of people moving into the area. In addition, because there are so many bear relocations to the same remote areas of Yellowstone, and because male bears tend to dominate the center of the relocation zone, female bears tend to be pushed to the boundaries of the region and beyond. The result is that a large proportion of repeat offenders, bears that are killed for public safety, are females. This creates a further depressive effect on an already endangered species. The grizzly bear is officially described as threatened in the U.S. Though the problem is most significant with regard to grizzlies, these issues affect the other types of brown bear as well.

In Europe, part of the problem lies with shepherds; over the past two centuries, many sheep and goat herders have gradually abandoned the more traditional practice of using dogs to guard flocks, which have concurrently grown larger. Typically they allow the herds to graze freely over sizeable tracts of land. As bears reclaim parts of their range, they may eat livestock. The shepherd is forced to shoot the bear to protect his livelihood.

There are an average of two fatal attacks a year in North America. In Scandinavia, there are only four known cases during the last 100 years in which humans were killed by bears. Attacks usually occur because the bear is injured or a human encounters a mother bear with cubs. Some types of bears, such as polar bears, are more likely to attack humans when searching for food, while American black bears are much less likely to attack.