Pediatric Hematology Oncology as a specialty was possible because of the evolution of the science of Hematology, which developed microscopy for describing blood cell morphology and methods for quantitation of these elements. Before pediatric blood diseases could be defined, it was necessary to establish the normal blood values of infancy and childhood. The unique features of the blood of the newborn were the focus of many of the early studies. After normal values were established, specific blood disease and hematologic syndromes of children began to be described in Europe and the United States. Pediatric Hematology Oncology is a broad and complex area that encompasses perturbations of the several-formed elements of the blood and their precursors in the bone marrow, as well as the coagulation-fibrinolytic systems in the plasma, the reticuloendothelial system, and malignancies of the blood and solid tissues and organs. The interactions of the blood and nutrition have long been important areas of study. Advances in Pediatric Oncology have been particularly spectacular in the last 50 years. Using multi-modal therapy including combination chemotherapy, more than 80% of children with cancer can now be cured. During the last 50 years, Pediatric Hematology Oncology has increasingly used tools of the "new biology": immunology, biochemistry, enzymology, genetics and molecular genetics, and others. During the last century, many diseases have been recognized and defined by biochemical and genetic mechanisms, and in some instances they have been prevented or cured.