Calcium ions are the probably the most ancient, the most universal and omnipresent intracellular signalling molecules, which are involved in regulation of a host of cellular functional reactions. In the nervous system Ca²⁺ signalling is intimately involved in information transfer and integration within neural circuits. Local Ca²⁺ signals or Ca²+ microdomains control neurotransmitter release; more global Ca²⁺ signals regulate synaptic strength and accomplish postsynaptic processing. In the glial syncytium Ca²⁺ ions provide for glial "Ca²⁺ excitability", convey long-range signalling by means of propagating Ca2+ waves and control the release of gliotransmitters. Differential Ca²⁺ signals in various elements of neural circuits represent therefore molecular mechanisms of integration in the nervous system.