The auditory evoked potential (AEP) components with latencies less than 10 ms, commonly called "brainstem auditory evoked potentials" (BAEPs), are the most useful AEPs for intraoperative monitoring (IOM). Anesthetics produce only minor changes in them. During IOM, each patient serves as his or her own control; BAEPs recorded when the ears or parts of the auditory pathways are at risk are compared to those recorded earlier during the same operation. Monaural stimuli should be used, but right and left ear stimuli may be interleaved, averaging BAEPs to left and to right ear stimulation concurrently. BAEPs are most often recorded between the vertex and the earlobes or mastoids ipsilateral and contralateral to the stimulated ear. Near-field AEPs can also be recorded from the proximal eighth nerve when feasible. Both amplitudes and latencies should be assessed during IOM of BAEPs. BAEP wave I is generated in the distal auditory nerve (at its cochlear end). Wave II has two generators, in the distal auditory nerve and in the region of the cochlear nucleus. Subsequent BAEP components are composites of contributions of multiple generators. Wave III predominantly reflects activity in the caudal pontine tegmentum, around the superior olivary complex; wave V predominantly reflects activity at the level of the inferior colliculus. In patients with asymmetrical or unilateral lesions, BAEP abnormalities are usually most pronounced following stimulation of the ear ipsilateral to the lesion. BAEPs cannot be used to assess or monitor the auditory pathways rostral to the mesencephalon. Causes of adverse BAEP changes include auditory pathway compromise from compression or traction, thermal injury from cauterization, and ischemia due to compromise of the vascular supply to the tissue. The pattern of BAEP changes can provide information about the location of the dysfunction. Several possible causes of intraoperative BAEP changes are described in this chapter, including some that are artifactual.