Glial fibrillary acid protein (GFAP) is a member of the type III intermediate filament family of proteins. GFAP is heavily expressed in astrocytes and certain other astroglia in the central nervous system, in satellite cells in peripheral ganglia, and in non-myelinating Schwann cells in peripheral nerves. It is closely related to its non-epithelial family members, vimentin, desmin, and peripherin, which are all involved in the structure and functions of the cell’s cytoskeleton. GFAP is thought to help to maintain astrocyte mechanical strength, as well as the shape of cell; however, its precise function remains poorly understood. In adults, GFAP levels increase in response to the proliferation of astrocytes associated with Alzheimer’s disease, epilepsy, and multiple sclerosis. Antibodies specific for GFAP are useful as markers of astrocytic cells and neural stem cells as well as for distinguishing neoplasms of astrocytic origin from other neoplasms in the central nervous system.