Magnesium (Mg, Lat. magnesium) is a divalent chemical element widely distributed all over the world. In the human body magnesium is one of the macroelements, a cofactor for more than 30 enzymes. Proper levels of magnesium in human serum range from 0.75 to 0.95 mmol/l (1.8-2.3 mg/dl). In the paper we present the role of magnesium in the homeostasis of the body and discuss in detail the influence of its deficiency and excess on human health. We also present miscellaneous sources of magnesium and possibilities of its supplementation in the diet.