For thousands of years and across many cultures, mankind has held folk beliefs about the existence of the doppelganger, a German word meaning ""double walker."" According to these beliefs, everyone is thought to have a double, an identical second self that is often the antithesis of one's original identity and is usually considered an omen of misfortune or death. The theme of the double has inspired works by E.T.A. Hoffmann, Poe, de Maupassant, Dostoevsky and others, and has been the basis for classic films in the genres of mystery, horror and science fiction. This theme is examined in film noir and neo-noir, with a special emphasis on the works of Alfred Hitchcock. In the realm of horror, films by Mario Bava, Roger Corman, David Crobnenberg, George Romero and other masters of the macabre are discussed. The theme also appears in many science fiction classics, from Fritz Lang's Metropolis to James Cameron's Avatar. Over one hundred films by acclaimed directors such as Robert Siodmak, Don Siegel, John Frankenheimer, Terry Gilliam, Brian De Palma, Roman Polanski and others are analyzed in this critical survey of one of the most enduring and mysterious motifs in cinema history.