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.

Bears have been hunted since prehistoric times for their meat and fur. To this day, they play a prominent role in the arts, mythology, and other cultural aspects of various human societies. In modern times, the bear's existence has been pressured through the encroachment on its habitats and the illegal trade of bears and bear parts, including the Asian bile bear market. The IUCN lists six bear species as vulnerable or endangered, and even least concern species such as the brown bear are at risk of extirpation in certain countries. The poaching and international trade of these most threatened populations is prohibited, but still ongoing.

The family Ursidae is one of nine families in the suborder Caniformia, or "doglike" carnivores, within the order Carnivora. Bears' closest living relatives are the pinnipeds (seals and kin).

The following synapomorphic (derived) traits set bears apart from related families:

Additionally, members of this family possess posteriorly oriented M2 postprotocrista molars, elongated m2 molars, and a reduction of the premolars.

Modern bears comprise eight species in three subfamilies: Ailuropodinae (monotypic with the giant panda), Tremarctinae (monotypic with the Spectacled Bear), and Ursinae (containing six species divided into one to three genera, depending upon authority).

The earliest members of Ursidae belong to the extinct subfamily Amphicynodontinae, including Parictis (late Eocene to early middle Miocene, 38-18 million years (Ma) ago) and the slightly younger Allocyon (early Oligocene, 34-30 Ma), both from North America. These animals looked very different from today's bears, being small and raccoon-like in overall appearance, and a diet perhaps more similar to that of a badger. Parictis does not appear in Eurasia and Africa until the Miocene. It is unclear whether late Eocene ursids were also present in Eurasia, although faunal exchange across the Bering land bridge may have been possible during a major sea level low stand as early as the late Eocene (~37 Ma) and continuing into the early Oligocene. European genera morphologically very similar to Allocyon, and also the much younger American Kolponomos (~18 Ma), are known from the Oligocene, including Amphicticeps and Amphicynodon.

The raccoon-sized, dog-like Cephalogale is the oldest-known member of the subfamily Hemicyoninae which first appeared during the middle Oligocene in Eurasia ~30 Ma ago. The subfamily also includes the younger genera Phoberocyon (~20-15 Ma), and Plithocyon (~15-7 Ma).

A Cephalogale-like species gave rise to the genus Ursavus during the early Oligocene (30-28 Ma); this genus proliferated into many species in Asia and is ancestral to all living bears. Species of Ursavus subsequently entered North America together with Amphicynodon and Cephalogale during the early Miocene (21-18 Ma).