The parasympathetic innervation to the cerebral vessels has only recently begun to be investigated in detail and to be understood in terms of the control of brain function. Much of the experimental work has been motivated by the potent vasoactive potential of the parasympathetic fibres, which can produce profound increases in cerebral blood flow (CBF) when electrically stimulated. This paper first outlines the principal anatomical and biochemical evidence for the origins and pathways of the parasympathetic fibres and the basis of their influence on CBF. Next, studies performed to elucidate their physiological function are reviewed, and finally their possible functional significance is discussed in relation to normal and pathophysiological brain function and their possible involvement in the trigeminovascular system.