summary Monitoring depth of general anesthesia One of the objectives of modern anesthesia is to ensure adequate depth of anesthesia to prevent awareness without inadvertently overloading the patients with potent drugs. One of the achievements of modern anesthesia is the ability to monitor depth of anesthesia. Adequate surgical anesthesia must achieve three goals: immobility, amnesia and absence of awareness. After the evidence of anesthetic lipophilicity was presented it was assumed that all these actions were accomplished at some unitary site. A body of evidence has now accumulated demonstrating that for many anesthetic agents, the dose required to suppress consciousness exceeds the amnestic dose but is substantially less than that required for surgical immobility during noxious stimuli. This suggests that these three dimensions may be mediated by different regions of the central nervous system, the variability among anesthetics of the ratios of concentrations needed to suppress consciousness, to block memory, and to achieve surgical immobility further invalidate the unitary hypothesis. (John ER and prichep LS, 2005)