The polygraph, an apparatus that records the physical changes in the subject's body which accompany his emotional changes, is comprised of three units, one recording continuously and quantitatively the subject's blood pressure and pulse, one giving a duplicate blood-pressure pulse curve taken from some other part of the subject's body, and the third recording respiration. A diagnostic instrument for the detection of deception, a polygraph can work only in the hands of a trained and competent operator who interprets its results accurately. This article describes a case in which an innocent person was cleared of charges stemming from a theft and the guilty person was identified. The polygraph, which has saved police departments an inestimable amount of time and money, is useful in locating stolen property, identifying persons who have knowledge of a crime and can name the perpetrators, checking on fictitious reports of burglaries and other offenses, and testing witnesses who offer accounts believed to be false, prejudicial, or exaggerated. Many police agencies use the polygraph to screen potential recruits.