Glial cells are the most numerous cells in the human brain but for many years have attracted little scientific attention. Neurophysiologists concentrated their research efforts instead on neurones and neuronal networks, because it was thought that they were the key elements responsible for higher brain function. Recent advances, however, indicate this isn't exactly the case. Not only are astroglial cells the stem elements from which neurones are born, but they also control the development, functional activity and death of neuronal circuits. These ground-breaking developments have revolutionized our understanding of the human brain and the complex interrelationship of glial and neuronal networks in health and disease.