Calcium (Ca) is an easily available element in nature, nevertheless Ca deficiency may occur in soils with low base saturation or high acid content. Calcium cations (Ca2+) are taken up passively from the soil by roots and transported to the shoot through the xylem in a transpiration rate-dependent process, the long-distance transport of Ca2+ being predominantly apoplastic. Calcium transport across membranes is carried out by different channels and transporters, and its cytosolic efflux mainly occurs through H+/Ca2+ antiporters and Ca2+ ATPases. Calcium is a multifunctional macronutrient in vascular plants. In addition to its structural role in the cell wall and membranes, it is a counterion for inorganic and organic anions. Calcium also acts as a key second messenger in many processes controlling plant development and responses to biotic and abiotic stresses. Several Ca2+-related proteins are involved in signaling pathways, which are associated with the maintenance of an extremely low cytosolic Ca2+ concentration. Furthermore, Ca use efficiency varies with crop species. Usually, Ca accumulation in tissues is, on average, higher in leaves than in grains and fruits. In this chapter, all these aspects related to Ca are addressed in a comprehensive manner.