A doppelganger, also spelled doppelgaenger, can be the ghost of a living person or any other sort of physical double. The idea of a doppelganger is sometimes similar to that of an "evil twin." The word doppelganger comes from the German Doppelgaenger, literally meaning "double-goer."

There are many different types of doppelganger, as the definition of the term has become somewhat loose, encompassing any sort of double. The doppelganger may be ghostly or appear in the flesh. It may be an "evil twin" unknown to the original person who causes mischief by confusing friends and relatives, or it may be the result of the original person being in two places at once through an act of sorcery. Scientists at the University Hospital in Geneva, Switzerland discovered that electrical stimulation of the brain, used to treat epilepsy, can produce the sensation of a doppelganger's presence in the patient.

In folklore, the doppelganger is said to have no shadow or reflection, much like vampires in some traditions. Doppelgangers are often malicious or a bad omen, and they can haunt their earthly counterparts. They may also give bad advice or put thoughts in their victim's heads. Seeing one's own doppelganger or the doppelganger of a friend or relative is considered very bad luck, often heralding death or serious illness of the doppelganger's original.

Doppelgangers appear often in various types of fiction, from mistaken identity plots in novels and plays to more supernatural doppelganger phenomena in works of science fiction, fantasy, and horror. There are many famous accounts of doppelgangers in history as well. Guy de Maupassant's short story Lui (Him) tells of the writer's own experience with a doppelganger. English poet John Donne claimed to have met his wife's doppelganger in Paris shortly before his daughter was stillborn. Poet Percy Bysshe Shelley and President Abraham Lincoln both saw doppelgangers that presaged their death; Shelley in a dream and Lincoln in his mirror.