In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at specific times of the year, or at specific points in its life cycle.

In medieval times, it was also known as "mewing" (from the French verb "muer", to moult), a term that lives on in the name of Britain's Royal Mews where the King's hawks used to be kept during moulting time before becoming horse stables after Tudor times.

Moulting can involve shedding the epidermis (skin), pelage (hair, feathers, fur, wool), or other external layer. In some groups, other body parts may be shed, for example, the entire exoskeleton in arthropods, including the wings in some insects.

In birds, moulting is the periodic replacement of feathers by shedding old feathers while producing new ones. Feathers are dead structures at maturity which are gradually abraded and need to be replaced. Adult birds moult at least once a year, although many moult twice and a few three times each year. It is generally a slow process: birds rarely shed all their feathers at any one time. The bird must retain sufficient feathers to regulate its body temperature and repel moisture. The number and area of feathers that are shed varies. In some moulting periods, a bird may renew only the feathers on the head and body, shedding the wing and tail feathers during a later moulting period.

Some species of bird become flightless during an annual "wing moult" and must seek a protected habitat with a reliable food supply during that time. While the plumage may appear thin or uneven during the moult, the bird's general shape is maintained despite the loss of apparently many feathers; bald spots are typically signs of unrelated illnesses, such as gross injuries, parasites, feather pecking (especially in commercial poultry), or (in pet birds) feather plucking. Some birds will drop feathers, especially tail feathers, in what is called a "fright moult".

The process of moulting in birds is as follows: First, the bird begins to shed some old feathers, then pin feathers grow in to replace the old feathers. As the pin feathers become full feathers, other feathers are shed. This is a cyclical process that occurs in many phases. It is usually symmetrical, with feather loss equal on each side of the body. Because feathers make up 4–12% of a bird's body weight, it takes a large amount of energy to replace them.

For this reason, moults often occur immediately after the breeding season, but while food is still abundant. The plumage produced during this time is called postnuptial plumage. Prenuptial moulting occurs in red-collared widowbirds where the males replace their nonbreeding plumage with breeding plumage. It is thought that large birds can advance the moult of severely damaged feathers.

Determining the process birds go through during moult can be useful in understanding breeding, migration and foraging strategies. One non-invasive method of studying moult in birds is through using field photography. The evolutionary and ecological forces driving moult can also be investigated using intrinsic markers such as stable hydrogen isotope (δ2H) analysis. In some tropical birds, such as the common bulbul, breeding seasonality is weak at the population level, instead moult can show high seasonality with individuals probably under strong selection to match moult with peak environmental conditions.