BACKGROUND: The essential trace element iron, which can occur in various oxidation states, is required for many biochemical reactions and processes in the human body. METHODS: This review summarizes the current knowledge about the physiology of iron metabolism. RESULTS: The physiological functions comprise oxygen transport in the blood, electron transport processes, DNA synthesis and gene regulation, the regulation of cell growth and differentiation, and the energy production in mitochondria. The average daily requirement of approximately 1 - 2 mg iron must be covered with dietary intake in form of heme iron in meat and non-heme iron in vegetables and fortified cereal products. Dietary iron is absorbed in the duodenum and the proximal ileum. The hepcidin-mediated ferroportin on the basolateral membrane of the enterocytes regulates the iron transport into the blood circulation. The transferrin-bound iron in the blood undergoes an intracellular uptake via the transferrin receptor TfR1 and TfR2. Intracellular storage iron is physiologically mobilized from ferritin and hemosiderin mainly for erythropoiesis. An effective iron-recycling mecha-nism in the liver and spleen guarantees the permanent body's daily requirement. Controlled intestinal iron absorption and iron recycling are part of the main physiological mechanisms to keep the iron levels in the human body in balance. CONCLUSIONS: The physiological function of iron is diverse and complex. The daily required iron intake of 1 - 2 mg is regulated by the hepcidin-dependent protein ferroportin. Intracellular iron storage and a recycling mechanism of iron guarantee the continuous supply of this trace element to the human body.