The conditions for which blood transfusion may be used fall into four well-defined groups. On the one hand are those characterized by an acute anæmia, which demand the performance of a blood transfusion as an emergency or life-saving operation; on the other hand are those in which the anæmia is of slow onset, and is to be combated by a single transfusion to tide the patient over an operation or a critical period or by repeated transfusions in the hope of prolonging the patient’s life if not of obtaining a cure. A third group includes the hæmorrhagic diseases in which the transfusion is administered not only to replace blood which has been lost, but also to bring about cessation of the hæmorrhage. A fourth group includes cases of general toxæmia, whether chemical or bacterial, in which the new blood is given partly on account of its therapeutic properties, partly in order to dilute the circulating toxins or to supply healthy red blood cells to carry on the oxygenation of the tissues. For the first and third of these groups blood transfusion is now very firmly established as a method of treatment which is of extraordinary value. For the second group it may be regarded as a palliative to be given with circumspection. For the fourth group administration of blood is still in the experimental stage. In the present work each condition will be taken in turn and, as far as possible, separately, though at the outset it has been found undesirable to dissociate the two conditions, hæmorrhage and shock. The present position of blood transfusion in relation to each condition will be discussed; its limitations and the precautions to be observed will be described. HÆMORRHAGE AND SHOCK Blood transfusion is pre-eminently the best form of treatment that is known for the condition of acute anæmia following hæmorrhage to whatever cause it may be due. Its good effects were seen by a number of operators in many hundreds of exsanguinated patients during the latter part of the war, and its value was then established upon a secure foundation. It was unusual during the war to meet with patients who were in danger of their lives from loss of blood alone without the additional factor of traumatic shock, but such cases did occur, and they are also to be met with in civil practice, as, for instance, in attempted suicide by throat cutting, in gastric ulcer with severe hæmatemesis, and in secondary hæmorrhage after operation. The more typical condition following war wounds, hæmorrhage with shock, will be faithfully reproduced in the victims of train or street accidents, in patients who have undergone certain severe operations, and in women suffering from post-partum hæmorrhage or a ruptured ectopic gestation.