The scientific order Carnivora includes bears, dogs, cats, raccoons, otters, hyenas, weasels, and their relatives. All typical carnivores have well developed claws, strong facial musculature, and specialized teeth. The canines are long, sharp, and well-developed, and premolars are adapted for cutting.

All eight bear species belong to this family. Ursids are the largest of all terrestrial carnivores. The family is divided into three subfamilies, Ursinae (black bears, brown bears, polar bears, sun bears, and sloth bears), Tremarctinae (spectacled bears), and Ailuropodinae (giant pandas).

There are five other species in the genus Ursus: American black bears, Asiatic black bears, brown (grizzly) bears, sun bears, and sloth bears. Species can be distinguished by size, build, coloration, and habitat.

Ursus maritimus is Latin for "sea bear". The polar bear is the only bear considered to be a marine mammal. It is only partially aquatic though, and the least aquatic of all marine mammals.

Ursids in the fossil record date back to the early Miocene in Asia, and the late Miocene in North America.

The oldest known polar bear fossil is approximately 130,000 years old. Polar bears probably diverged about 200,000 years ago from an ancestral brown bear. Polar bears and brown bears are still closely related; when cross-bred, they produce fertile offspring.