Ca²⁺ is well established as an intracellular second messenger. However, the molecular identification of a detector for extracellular Ca²⁺ — the extracellular calcium-sensing receptor — has opened up the possibility that Ca²⁺ might also function as a messenger outside cells. Information about the local extracellular Ca²⁺ concentration is conveyed to the interior of many cell types through this unique G-protein-coupled receptor. Here, we describe new emerging concepts concerning the signalling function of extracellular Ca²⁺, with particular emphasis on the extracellular calcium-sensing receptor.