Although glial cells occupy about half of the brain's total volume, they have been less well studied than neurons and have been the subject of endless speculation regarding their role in the brain's work. An enormous amount of new information about glial cells has become available in recent years and has led to the realization that these cells interact importantly with neurons in the context of all of the brain's main functions. In most instances, the details of these complex interactions are still being worked out. We review some of the basic physiological properties of glial cells that are important in understanding many of their determined or proposed functions.