Deer are the ruminant mammals forming the family Cervidae. Male (and a few female) deer of all species grow and shed new antlers each year – in this they differ from permanently horned animals such as antelope – these are in the same order as deer and may bear a superficial resemblance.

For most deer the male is called a buck and the female is a doe, according to the size of the species. For many medium-sized deer the male is a stag and the female a hind, while for many larger deer the same words are used as for cattle: bull and cow. Terms for young deer vary similarly, with that of most being called a fawn and that of the larger species calf; young of the smallest kinds may be a kid. A group of deer of any kind is a herd. Usage of all these terms may also vary according to dialect.

Deer are widely distributed, and hunted, with indigenous representatives in all continents except Antarctica and Australia.

Deer live in a variety of biomes ranging from tundra to the tropical rainforest. While often associated with forests, many deer are ecotone species that live in transitional areas between forests and thickets (for cover) and prairie and savanna (open space). The majority of large deer species inhabit temperate mixed deciduous forest, mountain mixed coniferous forest, tropical seasonal/dry forest, and savanna habitats around the world. Clearing open areas within forests to some extent may actually benefit deer populations by exposing the understory and allowing the types of grasses, weeds, and herbs to grow that deer like to eat. Additionally, access to adjacent croplands may also benefit deer. However, adequate forest or brush cover must still be provided for populations to grow and thrive.

There are also several species of deer that are highly specialized, and live almost exclusively in mountains, grasslands, swamps, and "wet" savannas, or riparian corridors surrounded by deserts. Some deer have a circumpolar distribution in both North America and Eurasia.

The Eurasian Continent (including the Indian Subcontinent) boasts the most species of deer in the world, with most species being found in Asia. Europe, in comparison, has lower diversity in plant and animal species. However, many national parks and protected reserves in Europe do have populations of Red Deer, Roe Deer, and Fallow Deer. These species have long been associated with the continent of Europe. Historically, Europe's deer species shared their deciduous forest habitat with other herbivores such as the extinct tarpan (forest horse), extinct aurochs (forest ox), and the endangered wisent (European bison). Good places to see deer in Europe include the Scottish Highlands, the Austrian Alps, and the wetlands between Austria, Hungary, and Czech Republic. Spain, Eastern Europe, and the Caucasus Mountains still have virgin forest areas that are not only home to sizable deer populations but also for other animals that were once abundant such as the wisent, Eurasian Lynx, Spanish lynx, wolves, and Brown Bears.