Twelve years have passed since Berger1first described the electrical beat of the brain in man. In these twelve years workers all over the world have studied intensively the anatomic, physiologic and psychic correlates of the electroencephalogram.2More than six hundred articles dealing with this subject have been published and sufficient evidence has been accumulated to indicate the general significance of this new phenomenon and to show that it has practical value for the solution of clinical problems, and also that it has certain definite limitations. Before proceeding further, I shall define electroencephalography: It is the technic of recording the electrical beat of the brain. As Berger pointed out, this beat is analogous to the beat of the heart as it appears in the electrocardiogram. The brain beat is entirely electrochemical, however; it has no appreciable mechanical component. In the normal waking state the brain does not beat