Abstract: Calcium is an essential plant nutrient. Calcium deficiency may occur in acidic soils with low base saturation. Since the transpiration from enclosed tissues and fruit is low, Ca deficiency readily develops in fruit trees and vegetables with enclosed tissues or fruit. Calcium may traverse the root through an apoplastic or symplastic pathway. Calcium is also believed to be directly taken up by an actual fruit with a non-vascular bundle structure. At the molecular level, Ca enters plant cells through Ca2+-permeable ion channels in their plasma membranes, whereas low [Ca2+] in unstimulated cells is maintained by Ca2+-ATPase and Ca2+/H+ antiporters. The rapid influx of Ca2+ through cation channels in the plasma membranes, tonoplast and/or endoplasmic reticulum generates transient elevation and oscillation of [Ca2+]cyt that initiate cellular responses to a diverse range of developmental cues and environmental challenges. The cellular [Ca2+]cyt sensors include calmodulin, calcium-dependent protein kinases and calcineurin B-like proteins that allow plant cells to respond appropriately to [Ca2+]cyt signals. This article provides an overview of the mechanisms of Ca2+ transport across cellular membranes, the origins and specificity of [Ca2+]cyt signals and the characteristics of cellular [Ca2+]cyt sensors.