Astrocytes, the most prevalent type of cell found in the brain, are glial cells that play a critical role in the support and maintenance of homeostatic conditions in the central nervous system (CNS). In addition to regulating synaptic function and neuronal development, astrocytes contribute to CNS immune responses and regulate permeability of the blood-brain barrier. Astrocytes respond to, and become activated by, a variety of molecular events in the CNS including the appearance of abnormal molecules, the disappearance of signaling molecules that are typically present, and changes in extracellular levels of various cytokines and chemokines. Under normal physiological conditions, astrocytic activation can be advantageous, as it can help protect the CNS from pathogens or injury; however, if this activation is sustained for a prolonged period, it can lead to detrimental astrocyte reactivity. This current book provides new developments in astrocyte research. The first chapter reviews pathological roles of astrocytes in brain edema formation. Chapter Two discusses neuroimmune interactions of astrocytes with other CNS cell types. Chapter Three focuses on astrocytes in amyotrophic lateral sclerosis. The final chapter studies astrocyte's response to stress and injury.