Calcium ions are the most ubiquitous and pluripotent signalling molecules, which regulate a wide array of physiological and pathological reactions. The specific system, controlling cellular Ca2+ homeostasis appeared very early in the evolution, being initially survival system preventing Ca2+-mediated cell damage. Subsequently, the steep Ca2+ gradients maintained by Ca2+ homeostatic molecular cascades became the basis for Ca2+ signalling. This signalling system utilises Ca2+ channels and transporters localised in plasmalemma and intracellular membranes to create highly organised and compartmentalised cytosolic Ca2+ fluctuations occurring within the spatial and temporal domains. Changes in cytosolic Ca2+ concentrations regulate a multitude of Ca2+-dependent proteins, which serve as “Ca2+ sensors” and thus the effectors of Ca2+ signalling system.