Thirty-five years ago, when Stephen Kuffler and his colleagues at Harvard initiated a new era of research on the properties and functions of neuroglial cells, very few neuro scientists were impressed at the time with the hypothesis that neuroglial cells could have another, though more subtle, role to play in the nervous system than to provide static support to neurons. Today, very few neuroscientists are unaware of the fact that multiple interactions between neurons and glial cells have been described, and that they consti tute the basis for understanding the function and the pathology of the nervous system. Glial cells outnumber neurons and make up about one-half of the bulk of the nervous system. They are divided into two major classes: first, the macroglia, which include astrocytes and oligodendrocytes in the central nervous system, and the Schwann cells in the peripheral nervous system; and second, the microglial cells. These different classes of glial cells have different functions and contribute in different ways in the devel opment, function, and the pathology of the nervous system.