Cellular signaling mechanisms are designed to transmit information from the cell surface to specific targets within the cell. Usually, the information is transmitted by intracellular messengers, of which the calcium ion, Ca2+, is one of the most important. Generally, extracellular stimuli are converted in a transient increase in the cytosolic Ca2+ concentration, [Ca2+]c, which, in turn, activates cellular functions. The specificity of the activated signal is guaranteed by the complex spatial and temporal organization of the changes in Ca2+ concentration, which increases the versatility of thismessenger. Different cell types use distinct Ca2+ signals, as appropriate to their physiology. The ability to use Ca2+ in different modes helps cells to vary the amplitude, frequency, kinetics, and localization of the signal. It is now recognized that periodic [Ca2+]c changes (i.e., oscillations) probably represent the typical response of cells to physiological agonists concentrations.