It is a branch of medicine that deals with the morphology of blood and blood–forming tissues, and with their physiology and pathology. This science developed after Dutch microscopist, Antonie van Leeuwenhoek observed red corpuscles in blood and compared their size with that of a grain of sand. This path breaking discovery set up a trend for further discoveries and the complete development of this science. The 18th century physiologist, William Hewson, improved the description of the red corpuscles, studied the lymphatic system, and demonstrated the role of fibrin in the clotting of blood. The recognition of bone marrow as the producer of red blood cells, new methods of staining cells and more information about the signs and symptoms of pernicious anemia, leukemia, and a number of other disorders of the blood acted as major breakthrough in the field of Hematology. The 20th century opened with a new era in the science of Hematology. It saw great advances in this field. From accurate blood transfusions to the study of anemia’s, and from the role of food on blood production to the use of liver extracts. Thus, the scope of Hematology broadened further after the 2nd World War. Parallel discoveries in nutrition, biochemistry and all other fields of medicine saw Hematology flourish as an independent branch of medicine.