Adequate calcium intake is vital for infant health, and some cases of rickets have been associated with a low concentration of calcium in breastmilk. The concentration of calcium in breastmilk has been shown to vary widely both between mothers, and over the course of lactation. To address potential concerns about the adequacy of calcium intake for infants who are exclusively breastfed, we discuss the factors likely to be affecting the concentration of calcium in breastmilk. We review and provide new evidence for a physicochemical model of the interactions of calcium with other components of breastmilk, particularly phosphate, citrate and casein. A proposed mechanism for the control of the concentration of calcium in milk is described that highlights the influence of the concentrations of citrate and casein. Understanding these interactions clarifies why the concentration of calcium in breastmilk is not affected by manipulations of maternal dietary calcium and vitamin D.