The term “gliosis” is generally defined as the cellular process by which glial cells in the CNS respond to insult and is used to describe the functional, morphological, biochemical, and molecular changes that occur in response to injury or disease. However, gliosis is most associated with the activation of astrocytes in response to CNS insults and is therefore discussed here as reactive astrogliosis. Although the molecular, biochemical, and functional changes associated with reactive astrogliosis are not fully elucidated, the morphological changes are better described. One morphological hallmark is the up-regulation of the intermediate filament protein glial fibrillary acidic protein (GFAP) which is often accompanied by a thickening of the main astrocyte processes, or hypertrophy. In healthy tissue, GFAP is the main intermediate filament expressed and the expression depends upon the subpopulation of astrocytes examined. After CNS injury, the expression of GFAP is significantly increased albeit heterogeneity and regional differences remain. It is important to note that in both nonreactive and reactive astrocytes, the expression of GFAP protein that can be detected by immunohistochemistry (IHC) is limited to the proximal portions of cell processes which means that the complexity of the fine distal processes and their associated volume cannot be visualized with GFAP-IHC. Techniques for evaluating GFAP-IHC in brain and spinal cord tissue from rodents are discussed. It recently has been discovered that in healthy tissue, cortical, and hippocampal astrocytes are organized into adjacent, but nonoverlapping domains and that under some conditions of reactive astrogliosis this “tiling” of astrocyte processes can be lost. Astrocyte domain organization has been evaluated using diolistic labeling of cells in fixed slices and techniques for diolistic labeling to determine the domain organization of astrocytes using a gene gun system are detailed. Other techniques to measure reactive astrogliosis, including bioluminescence imaging, manganese-enhanced magnetic resonance imaging, electrophysiology of astrocyte inwardly rectifying potassium (Kir4.1) currents, evaluation of transcriptional control of the GFAP gene, and selective ablation of reactive astrocytes in a transgenic mouse model are overviewed.