Astrocytes (from Ancient Greek ἄστρον, ástron, "star" and κύτος, kútos, "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, regulation of cerebral blood flow, and a role in the repair and scarring process of the brain and spinal cord following infection and traumatic injuries. The proportion of astrocytes in the brain is not well defined; depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to around 40% of all glia. Another study reports that astrocytes are the most numerous cell type in the brain. Astrocytes are the major source of cholesterol in the central nervous system. Apolipoprotein E transports cholesterol from astrocytes to neurons and other glial cells, regulating cell signaling in the brain. Astrocytes in humans are more than twenty times larger than in rodent brains, and make contact with more than ten times the number of synapses.

Research since the mid-1990s has shown that astrocytes propagate intercellular Ca²⁺ waves over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca²⁺-dependent manner. Data suggest that astrocytes also signal to neurons through Ca²⁺-dependent release of glutamate. Such discoveries have made astrocytes an important area of research within the field of neuroscience.

Astrocytes are a sub-type of glial cells in the central nervous system. They are also known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. In humans, a single astrocyte cell can interact with up to 2 million synapses at a time. Astrocytes are classically identified using histological analysis; many of these cells express the intermediate filament glial fibrillary acidic protein (GFAP).

Several forms of astrocytes exist in the central nervous system: including fibrous (in white matter), protoplasmic (in grey matter), and radial.

The fibrous glia are usually located within white matter, have relatively few organelles, and exhibit long unbranched cellular processes. This type often has astrocytic endfeet processes that physically connect the cells to the outside of capillary walls when they are in proximity to them.

The protoplasmic glia are the most prevalent and are found in grey matter tissue, possess a larger quantity of organelles, and exhibit short and highly branched tertiary processes.^{[citation needed]}

The radial glial cells are disposed in planes perpendicular to the axes of ventricles. One of their processes abuts the pia mater, while the other is deeply buried in gray matter. Radial glia are mostly present during development, playing a role in neuron migration. Müller cells of the retina and Bergmann glia cells of the cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane.

Early assessments of energy use in gray matter signaling suggested that 95% was attributed to neurons and 5% to astrocytes. However, after discovering that action potentials were more efficient than initially believed, the energy budget was adjusted: 70% for dendrites, 15% for axons, and 7% for astrocytes. Previous accounts assumed that astrocytes captured synaptic K⁺ solely via Kir4.1 channels. However, it's now understood they also utilize Na⁺/K⁺ ATPase. Factoring in this active buffering, astrocytic energy demand increases by >200%. This is supported by 3D neuropil reconstructions indicating similar mitochondrial densities in both cell types, as well as cell-specific transcriptomic and proteomic data, and tricarboxylic acid cycle rates. Therefore "Gram-per-gram, astrocytes turn out to be as expensive as neurons".