Changes in the cytosolic concentration of calcium ions ([Ca(2+)]i) play a key second messenger role in signal transduction. These changes are visualized by making use of either Ca(2+)-sensitive fluorescent dyes or the Ca(2+)-sensitive photoprotein, aequorin. Here we describe the advances made over the last 10 years or so, which have conclusively demonstrated a second messenger role for [Ca(2+)]i in a few model plant systems. Characteristic changes in [Ca(2+)]i have been seen to precede the responses of plant cells and whole plants to physiological stimuli. This has had a major impact on our understanding of cell signaling in plants. The next challenge will be to establish how the Ca(2+) signals are encrypted and decoded in order to provide specificity, and we discuss the current understanding of how this may be achieved.