This chapter discusses pathophysiology of anaemias, which is a reduction in the concentration of haemoglobin in blood. When the blood haemoglobin is low, the red cell count and packed cell volume are also generally low. The anaemic person feels breathless and gets palpitation during exercise. Some clue to the cause of anaemia may be obtained from simple investigations such as peripheral smear, packed cell volume, red blood cell count and the haemoglobin level. Hypoplasia of the marrow may be due to irradiation or anticancer drugs, or due to some unknown factor. Anemia of any type stimulates erythropoietin production. Chronic renal disease is often associated with anaemia possibly because kidneys are the major source of erythropoietin. Haemoglobin synthesis is impaired in iron deficiency. Iron deficiency anaemia is microcytic and hypochromic in character. The situations in which anaemia forms a dominant part of the clinical picture are those associated with chronic blood loss.