Ca2+ is a ubiquitous second messenger in the signal transduction pathway(s) by which stomatal guard cells respond to external stimuli. Increases in guard-cell cytosolic free Ca2+ concentration ([Ca2+]cyt) have been observed in response to stimuli that cause both stomatal opening and closure. In addition, several important components of Ca2+-based signalling pathways have been identified in guard cells, including the cADP-ribose and phospholipase C/Ins(1,4,5)P3-mediated Ca2+-mobilizing pathways. The central role of stimulus-induced increases in [Ca2+]cyt in guard-cell signal transduction has been clearly demonstrated in experiments examining the effects of modulating increases in [Ca2+]cyt on alterations in guard-cell turgor or the activity of ion channels that act as effectors in the guard-cell turgor response. In addition, the paradox that Ca2+ is involved in the transduction of signals that result in opposite end responses (stomatal opening and closure) might be accounted for by the generation of stimulus-specific Ca2+ signatures, such that increases in [Ca2+]cyt exhibit unique spatial and temporal characteristics.