Calcium ions are essential factors controlling the balance between cell survival, growth, differentiation and metabolism. Ca2+ acts as a global second messenger involved in the regulation of all aspects of cell function. Fluctuations in the intra- and extracellular Ca2+ concentration [Ca2+] in response to different environmental stimuli drive most cellular functions. Therefore, sustenance of calcium homeostasis requires perfect organization in time and space that is achieved by calcium binding proteins (CaBPs). These proteins are involved in sensing and transforming calcium signals to downstream cellular responses. Growing number of evidence suggests than many human disorders, including cancer progression, are related to deregulation of cellular calcium homeostasis and defects in CaBPs functions. In this review we will focus on the roles of S100A proteins in intracellular and extracellular calcium signalling and homeostasis. The S100A subfamily is among the most distinctive of EF-hand CaBPs and are found exclusively in vertebrates. They are believed to have evolved to enable activation of specific biochemical pathways in parallel to the activity of Ca2+ sensors such as calmodulin and/or annexins. The importance of S100 proteins is underscored by their deregulated expression in neurodegenerative and inflammatory disorders, myopathies and cancer. In addition, S100 proteins serve as diagnostic markers in the clinic and are under constant investigation. Their roles and the roles of the S100A protein partners in normal and pathology will be also discussed.