Bile bears or battery bears are Asian black bears kept in captivity in China, South Korea, Laos and Vietnam to harvest bile, a digestive juice produced by the liver and stored in the gall bladder. When extracted, the bears’ bile is a valuable commodity for sale as an ingredient in traditional Chinese medicine (TCM). The bears are also known as moon bears because of the cream-colored crescent moon shape on their chest. The Asiatic black bear, the one most commonly used on bear farms, is listed as vulnerable on the World Conservation Union's (IUCN's) Red List of Threatened Animals.

To facilitate the bile milking process, the bears are commonly kept in extraction cages, also known as crush cages, that measure around 2.6 feet x 4.4 feet x 6.5 feet (79 cm x 130 cm x 200 cm) for an animal that weighs between 110 to 260 pounds (50 to 120 kg). While this allows for easier access to the abdomen, it also prevents the bears from being able to stand upright, or in some cases move at all. Living for 10–12 years under such circumstances results in severe mental stress and muscle atrophy. The Chinese media reported an incident in which a mother bear, having escaped her cage, strangled her own cub and then killed herself by intentionally running into a wall. In two model Chinese bile farms, the Humane Society of the United States (HSUS) reports that the bears are moved to the crush cages for milking, but the rest of the time live in a cage large enough to stand and turn around.

Bile or gall is a bitter-tasting, dark green to yellowish brown fluid, produced by the liver of most vertebrates, that aids the process of digestion of lipids in the small intestine. In many species, bile is stored in the gallbladder and upon eating is discharged into the duodenum. Bile is a composition of the following materials: water (85%), bile salts (10%), mucus and pigments (3%), fats (1%), inorganic salts (0.7%) and cholesterol (0.3%).

In the medical theories prevalent in the West from Classical Antiquity up to the Middle Ages, the body's health depended on the equilibrium between four "humors" or vital fluids: blood, phlegm, "yellow bile" (or choler) and "black bile". Excesses of the last two humors were thought to produce aggression and depression, respectively; and the Greek names for them gave rise to the English words "cholera" and "melancholia". Those same theories explain the derivation of the English word "bilious" from "bile", and the meaning of "gall" in English as "exasperation" or "impudence".

Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern Hemisphere and partially in the Southern Hemisphere. Bears are found in the continents of North America, South America, Europe, and Asia.

Common characteristics of modern bears include a large body with stocky legs, a long snout, shaggy hair, plantigrade paws with five nonretractile claws, and a short tail. While the polar bear is mostly carnivorous and the giant panda feeds almost entirely on bamboo, the remaining six species are omnivorous, with largely varied diets including both plants and animals.

With the exceptions of courting individuals and mothers with their young, bears are typically solitary animals. They are generally diurnal, but may be active during the night (nocturnal) or twilight (crepuscular), particularly around humans. Bears are aided by an excellent sense of smell, and despite their heavy build and awkward gait, they can run quickly and are adept climbers and swimmers. In autumn some bear species forage large amounts of fermented fruits which affects their behaviour. Bears use shelters such as caves and burrows as their dens, which are occupied by most species during the winter for a long period of sleep similar to hibernation.