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 behavioral development that may persist later in life, despite iron supplementation. On the other hand, iron is essential for the growth of bacteria, and is a pro-oxidant that can catalyze the formation of oxidative radicals that damage proteins, lipids and nucleic acids. Iron supplementation in iron-replete children may have adverse effects, e.g. increased risk of infections and impaired growth. Thus, the therapeutic range of iron is small, and prevention of both ID and iron overload is essential for optimal development. Aim of this thesis was to investigate the prevalence and risk factors of ID and iron deficiency anemia (IDA) in children living in a high-income country. Furthermore, we aimed to analyze the value of different iron status biomarkers in the diagnosis of ID. The studies described in this thesis focused on children who are thought to have an increased risk of ID, namely preterm infants (part I), healthy children aged 0.5 to 3 years (part II), and children with cystic fibrosis (CF) (part III).