A changeling, also historically referred to as an auf or oaf, is a human-like creature found in folklore throughout Europe. A changeling was believed to be a fairy that had been left in place of a human stolen by the fae folk.

A changeling is typically identifiable via a number of traits; 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, as well as 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 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, being 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 would also 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.

In medieval Scandinavia it was believed that trolls considered it more respectable to be raised by humans than by their own kind, and would consequently seize the opportunity to give their own children a human upbringing. Some believed that trolls would only take unbaptised children, since once a child had been baptized – and therefore received into the Catholic faith – the trolls were powerless to abduct it.

Beauty in human children and young women, particularly traits which evoke brightness or reflectivity, such as blonde hair and blue or silver eyes, are said to attract fairies, as they perhaps find preciousness in these perceived traits.

In Scottish folklore, the children might be replacements for fairy children in the tithe to Hell; this is best known from the ballad of Tam Lin. According to common Scottish myths, a child born with a caul (part of the amniotic membrane) across their face is a changeling, and will soon die (is "of fey birth").

Other folklore says that human milk is necessary for fairy children to survive. In these cases either the newborn human child would be switched with a fairy baby to be suckled by the human mother, or the human mother would be taken back to the fairy world to breastfeed the fairy babies. It is also thought that human midwives were necessary to bring fairy babies into the world.

Some stories tell of changelings who forget they are not human and proceed to live a human life. Changelings who do not forget, however, in some stories return to their fairy family, possibly leaving the human family without warning. The human child that was taken may often stay with the fairy family forever. Feeling connected to the fate of a changeling, there are families who merely turn their changeling loose to the wilderness.

Some folklorists believe that fairies were memories of inhabitants of various regions in Europe who had been driven into hiding by invaders. They held that changelings had actually occurred; the hiding people would exchange their own sickly children for the healthy children of the occupying invader.

In Ireland, looking at a baby with envy – "over looking the baby" – was dangerous, as it endangered the baby, who was then in the fairies' power. So too was admiring or envying a woman or man dangerous, unless the person added a blessing; the able-bodied and beautiful were in particular danger. Women were especially in danger in liminal states: being a new bride, or a new mother.

Putting a changeling in a fire would cause it to jump up the chimney and return the human child, but at least one tale recounts a mother with a changeling finding that a fairy woman came to her home with the human child, saying the other fairies had done the exchange, and she wanted her own baby. The tale of surprising a changeling into speech – by brewing eggshells – is also told in Ireland, as in Wales. Various legends describe other ways to foil a would-be fairy kidnapper. One was to shout "Gairim agus coisricim thú " (I bless you) or "God bless you," which would cause the fairy to abandon the child it was trying to steal. Another possible tactic was to insert oneself into an argument over who would keep the child; shouting out "Give it to me" would trick the fairy into releasing the child back to a human.

Changelings, in some instances, were regarded not as substituted fairy children but instead old fairies brought to the human world to die.

Irish legends regarding changelings typically follow the same formula: a tailor is the one who first notices a changeling, the inclusion of a fairy playing bagpipes or some other instrument, and the kidnapping of a human child through a window.

The modern Irish girl's name, Siofra, means an elvish or changeling child, deriving from Síobhra(í) meaning fairy(/fairies). The Aos sí, siabhra (commonly anglicised as "sheevra"), may be prone to evil and mischief. However, the Ulster folk song 'The Gartan Mother's Lullaby' also uses "sheevra" simply to mean "spirit" or "fairy".