Deer (singular and plural) are the ruminant mammals forming the family Cervidae. The two main groups are the Cervinae, including the muntjac, the elk (wapiti), the fallow deer and the chital, and the Capreolinae, including the reindeer (caribou), the roe deer and themoose.

Female reindeer, and male deer of all species (except the Chinese water deer), grow and shed new antlers each year. In this they differ from permanently horned antelope, which are in the same order, Artiodactyla.