Grey matter, or gray matter in American English, is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries.

[3] Recent studies using cross-sectional neuroimaging have shown that by around the age of 8 the volume of grey matter begins to decrease.

[6] The grey matter includes regions of the brain involved in muscle control, and sensory perception such as seeing and hearing, memory, emotions, speech, decision-making, and self-control.

[20][21][22] Women and men with equivalent IQ scores have differing proportions of grey to white matter in cortical brain regions associated with intelligence.

[23] Pregnancy renders substantial changes in brain structure, primarily reductions in grey matter volume in regions subserving social cognition.

[24] The profile of brain changes is comparable to that taking place during adolescence, a hormonally similar transitional period of life.

[25] In the current edition[26] of the official Latin nomenclature, Terminologia Anatomica, substantia grisea is used for English grey matter.