Cervidae is a family of hoofed ruminant mammals in the order Artiodactyla. A member of this family is called a deer or a cervid. They are widespread throughout North and South America, Europe, and Asia, and are found in a wide variety of biomes. Cervids range in size from the 60 cm (24 in) long and 32 cm (13 in) tall pudú to the 3.4 m (11.2 ft) long and 3.4 m (11.2 ft) tall Moose. Most species do not have population estimates, though the Roe deer has a population size of approximately 15 million, while several are considered endangered or critically endangered with populations as low as 200. One species, Père David's deer, is extinct in the wild, and one, Schomburgk's deer, went extinct in 1938.

The fifty-four species of Cervidae are split into nineteen genera within two subfamilies: Capreolinae (New World deer) and Cervinae (Old World deer). Extinct species have also been placed into Capreolinae and Cervinae. More than one hundred extinct Cervidae species have been discovered, though due to ongoing research and discoveries the exact number and categorization is not fixed.