Ca2+ is a ubiquitous second messenger in plant cell signalling. In this review we consider the role of Ca2+-based signal transduction in stomatal guard cells focusing on three important areas: (1) the regulation of guard cell turgor relations and the control of gene expression in guard cells, (2) the control of specificity in Ca2+ signalling, (3) emerging technologies and new approaches for studying intracellular signalling. Stomatal apertures alter in response to a wide array of environmental stimuli as a result of changes in guard cell turgor. For example, the plant hormone abscisic acid (ABA) stimulates a reduction in stomatal aperture through a decrease in guard cell turgor. Furthermore, guard cells have been shown to be competent to relay an ABA signal from its site of perception to the nucleus. An increase in the concentration of cytosolic free Ca2+ ([Ca2+]1) is central to the mechanisms underlying ABA-induced changes in guard cell turgor. We describe a possible model of Ca2+-based ABA signal transduction during stomatal closure and discuss recent evidence which suggests that Ca2+ is also involved in ABA nuclear signal transduction. Many other environmental stimuli which affect stomatal apertures, in addition to ABA, induce an increase in guard cell [Ca2+]1) This raises questions regarding how increases in [Ca2+]1) can be a common component in the signal transduction pathways by which stimuli cause both stomatal opening and closure. We discuss several mechanisms of increasing the amount of information contained within the Ca2+ signal, including encoding information in a stimulus-specific Ca2+ signal or Ca2+ signature', the concept of the 'physiological address' of the cell, and the use of other second messengers. We conclude by addressing the emerging technologies and new approaches which can be used in conjunction with guard cells to dissect further the molecular mechanisms of Ca2+-mediated signalling in plants.