A Dictionary of Hallucinations is designed to serve as a reference manual for neuroscientists, psychiatrists, psychiatric residents, psychologists, neurologists, historians of psychiatry, general practitioners, and academics dealing professionally with concepts of hallucinations and other sensory deceptions.

Our history is full of the reports of visionaries, prophets, and other figures who derived their insights and authority from what we would now call hallucinations. As these reports testify, individuals who knew how to deploy them convincingly for some noble cause were often rewarded with a high social status. Religious texts like the Bible even indicate that for a long time no event of importance was thought to take place without some announcement by a voice or vision from beyond. Thus one might hold that hallucinations have constituted a legitimate source of information and inspiration in most – if not all – ancient cultures. On the other hand, for other groups of individuals they have always constituted a significant source of suffering. But even in these cases, the concept of illness seldom seemed to come to mind. Those who needed help were more likely to be taken to a priest than to a physician. According to Zilboorg and Henry, for thousands of years it was unthinkable that doctors, with their earthly methods, would involve themselves in matters pertaining to the spirit. This may well have been the principal reason why biomedicine became involved in the study of hallucinations so late in its developmental history.