Iron deficiency occurs when requirements exceed intake and utilisation, or when iron is lost from the body. Iron deficiency is a common cause of anaemia and vice versa, but the two conditions are not synonymous. Iron deficiency remains a common problem particularly affecting pre-school children, adolescent girls, women of reproductive age, and women during pregnancy. Iron deficiency is also an important presentation of gastrointestinal bleeding, menorrhagia, and malabsorption including in coeliac disease. Iron deficiency may also occur when there are medications or disorders of the stomach that impair appropriate acidification of stomach contents. An under-recognised cause of iron deficiency has been blood loss through donation. Iron deficiency may cause fatigue, impaired cognitive development in children, reduced physical exercise performance, and neurological disorders such as restless leg syndrome. Appropriate diagnosis of iron deficiency requires utilisation of a range of biomarkers: some established, including ferritin and transferrin saturation, some well characterised but difficult to deploy such as soluble transferrin receptor, and some that are now entering widespread use such as reticulocyte haemoglobin. The master regulator of systemic iron utilisation, hepcidin, is also emerging as a clinically useful biomarker of iron need. In all patients diagnosed with iron deficiency, it is critical to consider the underlying cause; this may include a need for gastrointestinal endoscopy in some cases. Treatment of iron deficiency includes correction of the underlying cause, as well as administration with oral or parenteral iron. Modern formulations of parenteral iron have revolutionised approaches for treating iron deficiency and are becoming first line in some settings, despite their much higher cost. Specialised iron clinics may facilitate rapid treatment of iron deficiency within the outpatient or ambulatory setting.