The brain stem auditory evoked potentials (BAEP) or, more appropriately, the auditory brain stem response (ABR) is a series of 5-7, vertex-positive, time-locked electric events. They are usually aroused by a brief, rapid onset, high intensity acoustic click or tonal pip. The vertex positive nature of the events is dictated by the placement of surface electrodes, usually featuring an active electrode on the vertex, a referent electrode on the mastoid or earlobe ipsilateral to the signal source, and a ground electrode on the forehead or other neutral tissue. The size of the ABR is sufficiently small, often fractions of a microvolt, to require the use of many signal presentations (1000-2000 stimuli) and digital averaging of the resulting electrical activity for the stimulus time-locked potentials to be elicited from other physiologic "noise." Since the auditory brain stem potentials are known to be elicitable near a subject's behavioral threshold for the stimulus used, and since the various potentials are thought to originate from different points in the brain stem, they have proved useful in hearing evaluations of human beings for two purposes: a) they are useful as a means of measuring hearing sensitivity from patients who cannot or will not give accurate voluntary responses; and b) they are useful as a diagnostic tool for determining the probable cause of an auditory disorder.