Ca2+ is a universal second messenger used to regulate a wide range of cellular processes such as fertilization, proliferation, contraction, secretion, learning and memory. Cells derive signal Ca2+ from both internal and external sources. The Ca2+ flowing through these channels constitute the elementary events of Ca2+ signalling. Ca2+ can act within milliseconds in highly localized regions or it can act much more slowly as a global wave that spreads the signal throughout the cell. Various pumps and exchangers are responsible for returning the elevated levels of Ca2+ back to the resting state. The mitochondrion also plays a critical role in that it helps the recovery process by taking Ca2+ up from the cytoplasm. Alterations in the ebb and flow of Ca2+ through the mitochondria can lead to cell death. A good example of the complexity of Ca2+ signalling is its role in regulating cell proliferation, such as the activation of lymphocytes. The Ca2+ signal needs to be present for over two hours and this prolonged period of signalling depends upon the entry of external Ca2+ through a process of capacitative Ca2+ entry. The Ca2+ signal stimulates gene transcription and thus initiates the cell cycle processes that culminate in cell division.