Calcium is recognized as a ubiquitous cellular regulator and changes in cytosolic Ca²⁺ levels are known to be involved in plant processes as diverse as response to cold shock, hormone action, and touch signaling. The specificity of subsequent cellular response appears to be encoded, at least in part, in the temporal and spatial dynamics of the Ca²⁺ change and the spectrum of Ca²⁺-responsive proteins expressed in the cell. In this chapter we review the evidence for the informational content of the Ca²⁺ signal in plant cells and discuss our current knowledge of the channel and pump systems that shape these Ca²⁺ changes. We will also use the Ca²⁺-dependent Nod-factor signaling system that underlies rhizobium:plant interactions to highlight our current understanding of how the Ca²⁺ change may be transduced to the appropriate cellular response. In addition we use the example of Ca²⁺ uptake from soil and its subsequent translocation through the plant to show how, in addition to its role in cell signaling, this ion can act as a long-range messenger, integrating the cellular activity of the stomatal guard cell with water and nutrient uptake activities in the root. The theme that emerges from these examples is that although Ca²⁺ is involved in regulating a remarkably diverse array of plant processes, the more we understand about the systems that generate and respond to these Ca²⁺ changes, the more we realize just how sophisticated the molecular networks that encode and decode them turn out to be.