For many decades after their discovery, astrocytes, the abundant glial cells of the brain, were believed to work as a glue, supporting the structure and metabolic functions of neurons. A revolution that started over 30 years ago revealed many additional functions of these cells, including neurogenesis, gliosecretion, glutamate homeostasis, assembly and function of synapses, neuronal metabolism with energy production, and others. These properties have been confirmed, limited however, to proliferating astrocytes. During their aging or following severe brain stress lesions, proliferating astrocytes are converted into their no-longer-proliferating, senescent forms, similar in their morphology but profoundly modified in their functions. The changed specificity of senescent astrocytes is largely due to their altered gene expression. The ensuing effects include downregulation of many properties typical of proliferating astrocytes, and upregulation of many others, concerned with neuroinflammation, release of pro-inflammatory cytokines, dysfunction of synapses, etc., specific to their senescence program. The ensuing decrease in neuronal support and protection by astrocytes induces the development, in vulnerable brain regions, of neuronal toxicity together with cognitive decline. Similar changes, ultimately reinforced by astrocyte aging, are also induced by traumatic events and molecules involved in dynamic processes. Senescent astrocytes play critical roles in the development of many severe brain diseases. The first demonstration, obtained for Alzheimer’s disease less than 10 years ago, contributed to the elimination of the previously predominant neuro-centric amyloid hypothesis. The initial astrocyte effects, operating a considerable time before the appearance of known Alzheimer’s symptoms evolve with the severity of the disease up to their proliferation during the final outcome. Involvement of astrocytes in other neurodegenerative diseases and cancer is now intensely investigated.