Stomatal aperture is regulated by the stress-induced phyto-hormone abscisic acid (ABA), and by CO2 and Ca2+ concentrations. Hubbard et al. review the topic and propose models to explain the specificity of signalling for the common intermediate, intracellular Ca2+. These models are put in the context of recent research identifying ABA receptors. New genetic Ca2+-dependent protein kinase phenotypes and Ca2+ imaging analysis are presented in reference to these models, and the importance of measuring and tracking individual stomatal pore responses is introduced.