Since the review written by Hill1 in 1922, hundreds of contributions to the literature of the blood have been made. Many of these are of great interest in a physiologic sense; a few are important from the point of view of clinical medicine; most of them add little if anything to our general understanding of the diseases of the blood. Though enough progress has been made in certain chemical problems to permit the formulation of well ordered syntheses, the clinical concepts of dis¬ eases of the blood in infants and children remain disorderly and impracticable.In writing the present review, the object has been to include par¬ ticularly articles of general physiologic interest and articles having to do with clinical problems in pediatrics. Case reports and descriptions of technical procedures have been alluded to only briefly; not because these are unimportant but because their interest usually lies in details which could not be included without making the report inordinately long. For the sake of clearness, the following plan, not intended as a classification of the blood diseases, has been followed.1. Physiologic Considerations.