Changes in the cytosolic concentration of calcium ions ([Ca2+](i)) play a k
ey second messenger role in signal transduction. These changes are visualiz
ed by making use of either Ca2+-sensitive fluorescent dyes or the Ca2+-sens
itive photoprotein, aequorin. Here we describe the advances made over the l
ast 10 years or so, which have conclusively demonstrated a second messenger
role for [Ca2+](i) in a few model plant systems. Characteristic changes in
[Ca2+](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 unders
tanding of cell signaling in plants. The next challenge will be to establis
h how the Ca2+ signals are encrypted and decoded in order to provide specif
icity, and we discuss the current understanding of how this may be achieved
.