Intracellular Ca(2+)-storage organelles are found in virtually all eukaryotic cells. They play an important role in the regulation of the cytosolic free Ca2+ concentration and, thereby, in the regulation of cellular activity. Ca(2+)-storage organelles consist, in the simplest model of a Ca2+ pump, of a Ca(2+)-storage protein and a Ca(2+)-release channel. The primary structure of these functionally important proteins of Ca(2+)-storage organelles is similar in different cell types and conserved through evolution. In contrast, their spatial arrangement and, thus, the architecture of Ca(2+)-storage organelles may vary dramatically from one cell type to another.