Among the earliest systemic observations of the physiologic effects of anesthetic agents was John Snow’s description, in 1847, of the various stages of ether anesthesia. Although the focus has evolved somewhat, our interest in measuring depth of anesthesia has persisted. The concern was initially to avoid the hazards of overdose, and the prevention of underdosage. There is considerable interest in preventing potentially hazardous hemodynamic and movement responses and in preventing recall. Until recently, anesthesiologists lacked the ability to monitor the effects of anesthetics on the brain in term of depth or adequacy of anesthesia. Depth of anesthesia is difficult to define, because increasing anesthetic concentration is associated with such diverse phenomena as amnesia and loss of cognitive ability, in balance against the intense arousal that surgical stimulation can induce.The problem of unexpected awareness has concerned patients and anesthesiologists since the administration of general anesthesia. Three causes were discussed : light anesthesia, increased anesthetic requirements of some patients and machine malfunction or misuse. These causes are associated with responses and problems like movements, autonomic responses and other clinical responses.Measuring the depth of anesthesia is our interest in this thesis.1-Clinical measures: related to the movement response and the MAC concept.2-Electrophysiologic approaches using the Evoked responses, Isolated forearm technique, Spontaneous facial electromyogram, Spontaneous lower esophageal contractions, Spontaneous EEG and power spectral measures. 3-Bispectral EEG signal: which represents the successful effort to modal EEG versus behavioral responses and it has recently been used as a surrogate measure of anesthetic effects on the brain. It indicates both the potential for awareness and of relative hypnotic overdose.At the end, management and prevention of awareness during anesthesia should be feasible and learned as it carries with it a medicolegal consideration. We suggest hat “Not only must we not give too much anesthesia, we must not give too little”.