It is now known that astrocytes are not merely supporting cells but they also play an important role in neuronal functions. Astrocytes tightly ensheath neuronal synapses and regulate the excitation of neurons by the uptake of neurotransmitters; regulate the cerebral blood flow, cerebral fluid volume and extracellular concentrations of ions. They also supply fuel in the form of lactate and provide free radical scavengers such as glutathione for active neurons. These facts indicate that impaired function of astrocytes may lead to neuronal dysfunction. After brain injury (stroke, trauma or tumors) astrocytes are swollen and release active molecules such as glutamate or free radicals resulting in neuronal dysfunction. Thus, investigation of the molecular mechanisms of astrocyte function may reveal novel targets for the development of therapeutic tools in neuronal diseases.