Musk deer can refer to any one, or all seven, of the species that make up Moschus, the only extant genus of the family Moschidae. Despite being commonly called deer, they are not true deer belonging to the family Cervidae, but rather their family is closely related to Bovidae, the group that contains antelopes, bovines, sheep, and goats. The musk deer family differs from cervids, or true deer, by lacking antlers and preorbital glands also, possessing only a single pair of teats, a gallbladder, a caudal gland, a pair of canine tusks and—of particular economic importance to humans—a musk gland.

Moschidae is a family of pecoran even-toed ungulates, containing the musk deer (Moschus) and its extinct relatives. They are characterized by long 'saber teeth' instead of horns, antlers or ossicones, modest size and a lack of facial glands. While various Oligocene and Miocene pecorans were previously assigned to this family, recent studies find that most should be assigned to their own clades, although further research would need to confirm these traits. As a result, Micromeryx, Hispanomeryx, and Moschus are the only undisputed moschid members, making them known from at least 18 Ma. The group was abundant across Eurasia and North America during the Miocene, but afterwards declined to only the extant genus Moschus by the early Pleistocene.