Book excerpt: Hallucinations, a natural phenomenon as old as mankind, have a surprisingly wide range. They appear under the most diversified conditions, in the "normal" psyche as well as in severe chronic mental derangement. As a symptom, hallucinations are a potential part of a variety of pathological conditions in almost all kinds of psychotic behavior. In addition, lately, various psychological and sociological circumstances seem to favor widespread use and abuse of hallucinogens, substances able to produce hallucinations in the normal brain. They not rarely lead to serious psychopatho logy such as toxic, and mobilized or aggravated endogenous psycho ses. While such development adds to our scientific knowledge, it also contributes to our current social troubles. Neurologists and neuro-surgeons, psychiatrists, psychologists and other specialized researchers constantly have been dealing with the phenomenon, its roots and branches, and yet, its primary mechanisms are largely un known. However, investigators of hallucinations now seem to enter common ground on which meaningful discussions and joint approaches become feasible and more promising. We have come a long way from the Latin term "hallucinari", meaning to talk nonsense, to be absent-minded, to the modern con cept of "hallucinations". While the Latin word was descriptive of what may be due to hallucinations, the modern concept defines hal lucinations as subjective experiences that are consequences of men tal processes, sometimes fulfilling a purpose in the individual's mental life.