A well controlled brain water homeostasis is critical for normal brain function. In a macroscopic perspective the production and absorption of cerebrospinal fluid (CSF) needs to be in balance. On the microscopic level the regulation of water transport between cellular compartments and extracellular space is of utmost importance for cell volume regulation and optimal synaptic activity. Perturbations in water and ion homeostasis accompany brain injuries and are probably involved in several brain pathologies. Astrocytes are key players in maintaining brain water homeostasis as well as in metabolism. In the brain astrocytes express aquaporin 4 (AQP4). Astrocyte AQP4 water permeability can be dynamically regulated by endogenous and exogenous factors. AQP4 is thought to facilitate the critical astrocyte function of potassium homeostasis and is involved in pathological disorders such as brain edema. AQP1 is expressed in choroid plexus and is involved in CSF production. Reports on the expression and functional role of other aquaporins have been conflicting. The role of astrocytes and aquaporins in brain water and ion homeostasis will be discussed, including recent findings on functional and molecular interactions of AQP4 from our group.