Hallucination was always the ghost story’s elephant in the room. Even before the vogue for psychical research and spiritualism began to influence writers at the end of the nineteenth century, tales of horror and the supernatural, of ghosts and demons, had been haunted by the possibility of some grand deception by the senses. But what is certainly true is that, during the nineteenth century, hallucination took on a new force and significance not just in ghost stories and horror fiction, but in other forms of writing. Authors began to encourage their readers to assess whether the ghostly had its origins in some supernatural phenomenon from beyond the grave, or from some deception within our own minds. This wide-ranging book explores the many factors which contributed to this rise in the interest in hallucination and visionary experience, during the nineteenth century and beyond. Through a series of close and often unusual readings of numerous writers including Robert Louis Stevenson, Henry James, and Arthur Machen, this original study explores what happened when hallucination appeared in fiction, and – even more importantly – why it happened at all.