Hypnosis is defined as a method of producing an Altered State of Consciousness (ASC). In a controlled ASC, such as that produced by hypnosis, data can be either programmed into or elicited from the unconscious memory bank. Hypnotists and their direct predecessors have historically made major contributions to the development of all the recognized professions, such as medicine, chemistry, law, dentistry, and psychoanalysis. Problems hypnosis and all disciplines employing controlled ASC's manifest themselves in Residual Posthypnotic Effect (RPHE). Failures to recover from the RPHE arise from five types of situations: resistance to hypnosis, misunderstandings, resentment on the part of the subject, the unresolved conflict syndrome, and mismanagement. Basic to hypnosis is the progressive relaxation exercise that physically relaxes the client while allowing him to maintain a high level of conscious awareness. The hypnotists's two most valuable assets are his voice, which should have a soothing, persuasive quality, and his skill at suggestion and formulating affective ideas or phrases. Relaxation, fixation, visualization, and shock/confusion are the four basic techniques of hypnotic induction. Other techniques include levitation, the sleep-nerve method, deepening techniques, counting, quiet, pressure, arrested attention, and music. Both regression and the use of Chevruel's pendulum are or could be highly successful investigative tools, especially in cases with psychological overtones. The hypnosis interview for criminal investigations is best conducted by someone other than those in charge of the investigation since this would prevent leading the witness. Tape recordings of hypnotic interviews should begin the moment the witness and the interviewer meet. Some objections to the use of hypnosis in investigations include nonbelief in hypnosis, fear of hypnotic effects, religious objections, lack of legal precedents, and possible confabulations on the part of the subject. For criminal trials, the hypnosis investigator must establish his credentials as a hypnosis-investigator and expert witness, as well as develop his techniques, avoid persuasive suggestions, and protect the evidence. Chapter references, diagrams, and a glossary are included.