A wet nurse is a woman who nurses a baby not her own. A wet nurse may be employed if the birth mother of a baby is unable to breast-feed her baby for a variety of reasons. Some reasons may include use of certain drugs (prescription or illegal), illness, or inoperative breasts. Also members of higher classes would have their children wet-nursed. This is an outgrowth of an old tradition -- noblewomen would not breast-feed, and could become pregnant again sooner, to ensure an heir.

Through the recent widespread availability of infant formula, wet nurses are not very common or needed in developed countries.

Though not widely known in developed countries, a woman who has never been pregnant may produce milk. Through frequent stimulation of the areolae and nipples, a woman may begin lactating and, therefore, be able to nurse. The use of a wet nurse is still a common practice in many developing countries.