Astrocytes are the major cell type in the mammalian brain. They provide a variety of supportive functions to their partner neurons in the central nervous system (CNS), such as neuronal guidance during development, and nutritional and metabolic support throughout life. Astrocytes have also been implicated in various pathological processes. Impairment of normal astrocyte functions during stroke and other insults can critically influence neuron survival. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is also influenced by astrocyte surface molecule expression and trophic factor release. Numerous studies have demonstrated that astrocytes are among the most functionally diverse group of cells in the CNS. Much of what we have learned about astrocytes is from in vitro studies and astrocyte culture is a useful tool for exploring the diverse properties of this cell type.