A changeling, also historically referred to as an auf or oaf, is a human-like creature found throughout much of European folklore. According to folklore, a changeling was a substitute left by a supernatural being when kidnapping a human being. Although sometimes it was a "stock" (a piece of wood made magically to resemble the kidnapped human), more often the changeling was a supernatural being made magically to look like the kidnapped human.

Supernatural beings blamed for stealing children included fairies, demons, trolls, nereids and many others. Usually, the kidnapped human was a child; but there were cases, particularly in Scandinavia and Ireland, where adults were taken.

Some modern scholars have argued these stories of replaced children originated as folklore explanations for autism or other developmental conditions.

A changeling is typically identifiable via several traits, which vary from culture to culture.

In Irish legend, a fairy child may appear sickly and will not grow in size like a normal child, and may have notable physical characteristics such as a beard or long teeth. They may also display intelligence far beyond their apparent years and possess uncanny insight. A common way that a changeling could identify itself is through displaying unusual behaviour when it thinks it is alone, such as jumping about, dancing or playing an instrument – though this last example is found only within Irish and Scottish legend.

A human child might supposedly be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often, it was thought that fairies exchanged the children. In rare cases, the very elderly of the fairy people would be exchanged in the place of a human baby so that the old fairy could live in comfort, coddled by its human parents. Simple charms such as an inverted coat or open iron scissors left where the child sleeps were thought to ward them off; other measures included a constant watch over the child.

Folklorist D. L. Ashliman proposes in his essay 'Changelings' that changeling tales illustrate an aspect of family survival in pre-industrial Europe. A peasant family's subsistence frequently depended upon the productive labour of each member, and it was difficult to provide for a person who was a permanent drain on the family's scarce resources. "The fact that the changelings' ravenous appetite is so frequently mentioned indicates that the parents of these unfortunate children saw in their continuing existence a threat to the sustenance of the entire family. Changeling tales support other historical evidence in suggesting that infanticide was frequently the solution selected."

Fairies were also believed to take adult humans, especially the newly married and new mothers; young adults were taken to marry fairies instead, while new mothers were often taken to nurse fairy babies. Often when an adult was taken instead of a child, an object such as a log was left in place of the stolen human, enchanted to look like the person. This object in place of the human would seem to sicken and die, to be buried by the human family, while the living human was among the fairies. Bridget Cleary is one of the most well-known cases of an adult thought to be a changeling by her family: her husband killed her, attempting to force the fairies to return his 'real' wife.