Microdomains of Ca(2+), which are formed at sites where Ca(2+) enters the cytoplasm either at the cell surface or at the internal stores, are a key element of Ca(2+) signalling. The term microdomain includes the elementary events that are the basic building blocks of Ca(2+) signals. As Ca(2+) enters the cytoplasm, it produces a local plume of Ca(2+) that has been given different names (sparks, puffs, sparklets and syntillas). These elementary events can combine to produce larger microdomains. The significance of these localized domains of Ca(2+) is that they can regulate specific cellular processes in different regions of the cell. Such microdomains are particularly evident in neurons where both pre- and postsynaptic events are controlled by highly localized pulses of Ca(2+). The ability of single neurons to process enormous amounts of information depends upon such miniaturization of the Ca(2+) signalling system. Control of cardiac cell contraction and gene transcription provides another example of how the parallel processing of Ca(2+) signalling can occur through microdomains of intracellular Ca(2+).