Deer, common name for certain hoofed, artiodactyl mammals, usually characterized by bony, often branching antlers that are shed and regenerated annually. Deer range through the Americas, Europe, Asia, and North Africa. The largest populations occur in mixed wooded and open land, although deer also live in swamps, on mountains, and on northern tundras. Deer species range in size from the European elk, or moose (see Elk), which may reach a shoulder height of 2.35 m (7.7 ft), to the South American pudu, which can be as little as 25 cm (10 in) high at the shoulder. The first deer appeared in the early Oligocene epoch in Asia, about 38 million years ago.

Deer commonly have lithe, compact bodies and long, powerful legs suited for rugged woodland terrain. They are also excellent swimmers. Their lower cheek teeth have crescent ridges of enamel, which enable them to grind a wide variety of vegetation. The animals are ruminants, or cud chewers, and have a four-chambered stomach. Nearly all deer have a facial gland, in front of each eye, that contains a strongly scented substance, or pheromone, used to mark its home range. The males of many species open these glands wide when angry or excited. All deer except the musk deer have a liver without a gallbladder. The musk deer, along with the Chinese water deer, also differs from other species in that it has no antlers and bears upper canines that have developed into tusks.