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We critically review the principles underlying processed electroencephalogram (EEG) monitors and recent studies validating their use in monitoring anesthetic depth.Depth of anesthesia is a theoretical construct to conceptualize anesthetic effects on the central nervous system as discrete or continuous phases or states. Clinical signs for assessing anesthetic depth are currently being supplemented by brain monitors. Their use may help to prevent insufficient anesthesia, which can lead to intraoperative awareness with recall, as well as anesthetic overdose, which may be associated with adverse events. Commercial and open-source brain monitoring indices are computed from frequency, entropy, or information theoretic analysis of the spontaneous or evoked EEG. These techniques are undergoing refinement to determine the best method for titrating anesthetics. Future depth-of-anesthesia monitors will benefit from current work aimed at correlating anesthetic effects to alterations in specific neural circuits.Current processed EEG monitors are limited by their calibration range and the interpatient variability in their dose–response curves. The next generation of depth-of-anesthesia monitors will require a greater understanding of the transformations of cortical and subcortical activity into EEG signals, the effects of anesthetics at a systems level, and the neural correlates of consciousness.