The great interest which has been taken in haematology in the past ten or fifteen years and which has led to a rational classification of abnormal blood conditions, and to not a few therapeutic discoveries such as the liver treatment of pernicious anaemia, has not been confined to adult medicine. In paediatrics great strides have been made not only with regard to the diseases of the blood, but also in connection with the physiological changes in the haematology of the normal child from birth until the adult type of haematopoiesis has been acquired. In the adult the dyshaematopoietic anaemias like pernicious anaemia and microcytic hypochromic anaemia, which is so common in women, are of greater importance than the haemolytic anaemias. In childhood, however, the haemolytic anaemias are relatively more important, although nutritional or dyshaemopoietic anaemias are, as in adults, of greater frequency. Haemolytic anaemia in childhood is chiefly found in the neo-natal period, when it is known as erythroblastosis foetalis. This is a not uncommon condition, and, as I shall show, not less than 29 cases were observed in the Royal Hospital for Sick Children, Glasgow, in three years. It is principally with erythroblastosis foetalis that I deal in this thesis, but additional papers - one on acholuric jaundice in childhood, with special reference to red cell morphology, and another on the red cell shape in conditions of acidosis and alkalosis - have been added