This review provides an historical and current perspective on astrocytes in the CNS, detailing their morphological and biochemical diversity, origins and transformation to reactive astrocytes in experimental lesions and clinical disease states which result in gliosis. Recent studies on reactive astrocytes have been facilitated by the availability of immunocytochemical probes for GFAP and other distinctive cellular components. The effects in vitro of 'growth factors' on astrocytic transformation are briefly considered. An understanding of astrocytic transformation has implications for neural transplantation.