In European folklore of the medieval and early modern periods, familiars (strictly familiar spirits, as "familiar" also meant just "close friend" or companion) were believed to be supernatural entities or spiritual guardians that would protect and/or assist witches and cunning folk in their practice of magic. According to records of the time, those alleging to have had contact with familiar spirits reported that they could manifest as numerous forms, usually as an animal, but sometimes as a human or humanoid figure, and were described as "clearly defined, three-dimensional... forms, vivid with colour and animated with movement and sound", as opposed to descriptions of ghosts with their "smoky, undefined form[s]".

When they served witches, they were often thought to be malevolent, but when working for cunning folk they were often considered benevolent (although there was some ambiguity in both cases). The former were often categorized as demons, while the latter were more commonly thought of and described as fairies. The main purpose of familiars was to serve the witch, providing protection for them as they came into their new powers.

Since the 20th century some magical practitioners, including adherents of the Neopagan religion of Wicca, use the concept of familiars, due to their association with older forms of magic. These contemporary practitioners use pets or wildlife, or believe that invisible versions of familiars act as magical aides.