Anesthetic depth has been defined as the probability of non-response to stimulation, calibrated against the strength of the stimulus. The similarity between sleep and anesthesia is widely argued that these brain states are actually different, sleep being readily reversible, whereas anesthesia is irreversible.The first section in this essay discusses clinical measures of anesthetic depth that derive directly from the patient. Each discussion begins with a review of the subjective clinical approaches used in anesthetic practice. The second section discusses several electrophysiologic approaches to assessing depth of anesthesia. The BIS® (Aspect Medical Systems Inc., Newton, MA, USA), A-line® (Alaris Medical Systems, Inc., San Diego, CA, USA) devices are designed to monitor the hypnotic state of the patient. There are also both spontane-ous EEG, Evoked potentials and Entropy (GE Healthcare, Helsinki, Finland).