It is widely known that electroencephalogram (EEG) shows dramatic changes with increase of the concentration of anesthetic. It is considered that volatile anesthetics (i. e. isoflurane, sevoflurane), barbiturates, propofol show anesthetic effect by potentiating GABAA receptor. Changing patterns of EEG by these anesthetics are quite similar. In light anesthetic level, high frequency with low amplitude waves are dominant. With increase of anesthetic concentration, waves in alpha range (8-13 Hz) become dominant. In deeper levels, powers in alpha range then become smaller and theta or delta powers become dominant. With further deeper levels, EEG waveform changes into specific pattern so-called "burst and suppression", and finally it becomes flat. The author considers that prominent alpha power indicates adequate anesthetic level. However this is not always the required condition for adequate anesthesia, because alpha power never becomes larger in some patients even when the anesthetic level was judged as adequate by concentration dependent changing patterns of EEG. As EEG changes in relation to the concentration of anesthetic, it seems to be correlated with the level of consciousness. But EEG patterns during anesthesia are mainly determined by the condition of thalamic neurons, and it would merely indicate the level of hypnosis indirectly.