Musk is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial substances with similar odors. Musk was a name originally given to a substance with a strong odor obtained from a gland of the musk deer. The substance has been used as a popular perfume fixative since ancient times and is one of the most expensive animal products in the world. The name originates from the Late Greek μόσχος 'moskhos', from Persian mushk and Sanskrit मुष्क muṣka (lit. 'testicle') derived from Proto-Indo-European noun múh₂s meaning "mouse". The deer gland was thought to resemble a scrotum. The term is applied to various plants and animals of similar smell (e.g., muskox) and has come to encompass a wide variety of aromatic substances with similar odors, despite their often differing chemical structures and molecular shapes.

Natural musk was used extensively in perfumery until the late 19th century when economic and ethical motives led to the adoption of synthetic musk, which is now used almost exclusively. The organic compound primarily responsible for the characteristic odor of musk is muscone. There are several ways of preparing the commercial musk, and the best method is to dry the pod by sunning and airing immediately after it is taken from the animal. Natural musk is usually packed in hermetically-sealed vessels and wooden boxes lined with tin foil because of its powerful diffusion of odor.

Modern use of natural musk pods occurs in traditional Chinese medicine which, save for specially exempt drugs, uses a synthetic version of undisclosed composition created in 1994. The process was given State Science and Technology Progress Award First Class in 2015.

The musk deer belongs to the family Moschidae and lives in Tibet, India, Nepal, Pakistan, Afghanistan, China, Siberia, Mongolia, Manchuria, Korea and North Vietnam.

The musk pod, a preputial gland in a pouch, or sac, under the skin of the abdomen of the male musk deer, is normally obtained by killing the male deer through traps laid in the wild. Upon drying, the reddish-brown paste inside the musk pod turns into a black granular material called "musk grain", which is then tinctured with alcohol. The aroma of the tincture gives a pleasant odor only after it is considerably diluted. No other natural substance has such a complex aroma associated with so many contradictory descriptions; it is usually described abstractly as animalistic, earthy and woody or something akin to the odor of baby's skin.

Musk has been a key constituent in many perfumes since its discovery, being held to give a perfume long-lasting power as a fixative. Today, the trade quantity of the natural musk is controlled by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), but illegal poaching and trading continues.

Muskrat (Ondatra zibethicus), a rodent native to North America, has been known since the 17th century to secrete a glandular substance with a musky odor. A chemical means of extracting it was discovered in the 1940s, but it did not prove commercially worthwhile.

Glandular substances with musk-like odors are also obtained from the musk duck (Biziura lobata) of southern Australia, the muskox, the musk shrew, the musk beetle (Aromia moschata), the African civet (Civettictis civetta), the musk turtle (Sternotherus odoratus), the American alligator of North America, lynx musk, lungurion which, in antiquity, was highly valued, and from several other animals.