Brain function is inextricably coupled to water homeostasis. The fact that most of the volume between neurons is occupied by glial cells, leaving only a narrow extracellular space, represents an important challenge, as even small extracellular volume changes will affect ion concentrations and therefore neuronal excitability. Further, the ionic transmembrane shifts that are required to maintain ion homeostasis during neuronal activity must be accompanied by water. It follows that the mechanisms for water transport across plasma membranes must have a central part in brain physiology. These mechanisms are also likely to be of pathophysiological importance in brain oedema, which represents a net accumulation of water in brain tissue, and in epilepsy. Recent studies have shed light on the molecular basis for brain water transport and have identified a class of specialized water channels in the brain that are crucial for the physiological and pathophysiological handling of water. The lecture will focus on the predominant brain water channel - aquaporin-4 - and elucidate the role of this channel in normal brain function and disease.