The nervous system is built from two broad categories of cells, neurones and glial cells. The glial cells outnumber the neurones and the two cell types occupy a comparable amount of space in nervous tissue. The main glial cell types are, in the central nervous system, astrocytes and oligodendrocytes and, in the peripheral nervous system, Schwann cells, enteric glial cells and satellite cells. In the embryo, glial cells form a cellular framework that permits the development of the rest of the nervous system, and regulate neuronal survival and differentiation. The best known function of glia in the adult is the formation of myelin sheaths around axons thus allowing the fast conduction of signalling essential for nervous system function. Glia also maintain appropriate concentrations of ions and neurotransmitters in the neuronal environment. Increasing body of evidence indicates that glial cells are essential regulators of the formation, maintenance and function of synapses, the key functional unit of the nervous system.