This book is about hallucinations that occur in the context of drug use, sensory deprivation, torture, extreme fear, near-death experiences, dreams, and the like. Siegel has carved out a special niche in this area, having devoted his research, teaching and clinical and forensic career as a neuropsychiatrist to studying the phenomenon and trying to fathom the relationship of it to what is happening in the brain. No passive observer, he is himself an experienced ``psychonaut.'' Siegel presents 17 case studies, grouped under the headings of ``visionary drugs,'' ``dreams,'' ``imaginary companions,'' and ``life-threatening danger.''