The calcium ion is firmly established as a ubiquitous intracellular second messenger in plants. At their simplest, Ca2+-based signaling systems are composed of a receptor, a system for generating the increase in [Ca2+]cyt, downstream components that are capable of reacting to the increase in [Ca2+]cyt, and other cellular systems responsible for returning [Ca2+]cyt to its prestimulus level. Here we review the various mechanisms responsible for generating the stimulus-induced increases in [Ca2+]cyt known as Ca2+ signals. We focus particularly on the mechanisms responsible for generating [Ca2+]cyt oscillations and transients and use Nod Factor signaling in legume root hairs and stimulus-response coupling in guard cells to assess the physiological significance of these classes of Ca2+ signals.