The immunohistochemical localization of glial fibrillary acidic protein has proven its utility in the histologic identification of benign astrocytes and neoplastic cells of glial lineage in the central nervous system. Its application in the developing nervous system has contributed significantly to following the histogenesis of neural tissue. Its identification in various forms of injury and neoplasia has helped in discovering the function of astrocytes in these processes. In diagnostic neuropathology, its greatest contribution has been in the elucidation of the cellular constituencies of neoplasms previously considered of enigmatic origin.