Iron deficiency (ID) is the most common micronutrient deficiency in the world. Iron is involved in oxygen transport, energy metabolism, immune response, and plays an important role in brain development. In infancy, ID is associated with adverse effects on cognitive, motor and behavioural development that may persist later in life, despite iron supplementation. On the other hand, iron supplementation in iron-replete children may have adverse effects, e.g. increased risk of infections and impaired growth. Lieke Uijtershout investigates the prevalence and risk factors of ID and iron deficiency anemia in children living in a high-income country. She also analyses the value of different iron status biomarkers in the diagnosis of ID.