Vestibular information contributes to a number of brain functions. These include balance, gaze, navigation, self and non-self motion perception, voluntary movement, spatial orientation, and autonomic control. There is a major obstacle to isolating and studying the vestibular contribution to these brain functions. Any real movement or force that is applied to perturb the vestibular organs also evokes responses from many other sensory receptors making it difficult to extract the vestibular component. A way around this is to bypass the process of mechanical activation of the vestibular organs and perturb the vestibular system by stimulating behind the ears with small direct electrical currents. It turns out that this galvanic vestibular stimulation (GVS) technique has the same frequency-modulating effect on the vestibular afferents as natural movement, and is interpreted by the brain as such. With anatomical knowledge of the hair cell alignment in the vestibular organs, we can calculate the direction of the natural movement that would produce the same signal that GVS evokes (Fitzpatrick & Day, 2004). Vectorially summing the responses to GVS from the entire semicircular canal neurone population reveals a virtual rotation about an axis in the mid-sagittal plane of the head at an angle of 18.8 deg with respect to Reids plane. It is not as clear for the otolith organs but the vectorial sum suggests a small lateral acceleration. It is simply the idiosyncrasies of the vestibular anatomy that define these virtual head movements evoked by GVS. We have used this model of GVS to investigate vestibular influences on three different brain functions in healthy human subjects. A perceptual task involved judging the extent and direction of externally imposed body movement in the world. GVS revealed a process that transformed the vestibular signal from head to world coordinates and extracted the horizontal plane component of the total signal (Day & Fitzpatrick, 2005). A similar vestibular coordinate transformation process was found for a bipedal balance task, except in that case the extracted component was in the vertical plane. In addition, a short-latency otolith contribution to balance control was revealed (Cathers et al. 2005). A voluntary movement task consisted of a goal-directed movement of the upper body while GVS was applied. The instantaneous GVS current was determined by the angular velocity of the head. The resulting modification of the trajectory demonstrated that voluntary movements, in which the head is transported in space, are under the on-line control of vestibular reafference (Day & Reynolds, 2005).