Intraoperative neurophysiological monitoring is of benefit in protecting tissue at risk for trauma or ischemia during surgical procedures. Monitoring modalities include EEG, computer processed EEG, somatosensory (SEP), auditory (BAEP), and visual evoked potentials (VEP), and cranial nerve monitoring. The efficacy of monitoring is controversial, because no properly controlled prospective study of outcome with and without monitoring has been done. The weight of evidence suggests that loss of spontaneous EEG and SEP correlate well with critical reductions of cerebral blood flow. Meta-analysis of series comprising 3,028 patients undergoing carotid endarterectomies shows that SEP deteriorated in 5.6% of cases, with 20% of these having postoperative deficits, but more might have had deficits if they had not been shunted. SEP monitoring can be useful in surgery affecting brain and cord vasculature. Monitoring is not indicated for routine lumbosacral spine surgery. BAEPs have predictive value for preservation of hearing after acoustic neuroma surgery, and other surgery near the brainstem. VEPs have been too variable to be of major use in the operating room. For neurophysiologic monitoring to be useful, it must be performed by an experienced team, and the surgeon must be willing to act on the findings. Under these circumstances, monitoring can reduce surgical complications in selected cases.