The endoplasmic reticulum is not the only major agonist-releasable Ca2+ store within cells; it is now clear that virtually all organelles so far studied have the ability to act as mobilizable Ca2+ stores. From recent findings with regard to Ca2+ transportation and Ca2+ homeostasis within a variety of cell organelles such as the mitochondria, nucleus, Golgi and lysosomes, it emerges that many of these organellar Ca2+ stores appear to interact with each other, adding a further level of complexity to Ca2+ signalling events.