Lactation is the production of milk by female mammals. Shortly before the birth of her young, hormonal changes in the mother result in increased development of the mammary glands and teats. Glandular cells in the body of the mammaries secret milk which is released to the young when the teats are stimulated. Lactation and feeding of the young on milk are characteristic of the Mammalia.

Lactation begins in the first few days after childbirth. Immediately after delivery, the breasts produce a thick yellow fluid called colostrum, which provides the baby's first food. Sucking at the breat stimulates the release of the pituitary hormone oxytocin, which promotes the flow of milk.