The electroencephalographic signs of acute cerebrovascular accidents are mentioned. The "functional" nature of chronic cerebrovascular insufficiency is stressed. Because of the paucity of clinical signs of this condition, great reliance is given to its detection by polygraphic methods, notably electroencephalography and rheoencephalography. Signs of chronic cerebrovascular insufficiency are reviewed as they occur in resting electroencephalographic records, after inhalation of nitrogen and of a CO2-O2 mixture, and with compression of the carotids and of the carotid sinus. The value of rheonencephalography in allowing a functional evaluation of cerebral collateral circulation is pointed out. The advantages of these methods in confirming a diagnosis of carotid and basilai insufficiency and of carotid sinus hypersenitivity, in establishing the nature and site of a lesion, and in distinguishing ischemic from epileptic seizures are discussed.