Nuclear hematology deals with the use of radionuclides or radiopharmaceutical agents in the study of the pathophysiology, diagnosis, and therapy of hematological diseases. Radionuclides are now widely used to label the formed elements of the blood to trace their biological distribution, function, and lifespan in vivo as well as to study the proliferation and differentiation of hematopoietic progenitor and precursor cells in the bone marrow. The other major applications of nuclear hematology include the determination of spleen size, splenic sequestration of blood cells, and investigations relating to the absorption, metabolism, and utilization of hematopoietic nutrients such as iron, vitamin B12, and folate and bone marrow imaging. This chapter summarizes the important applications of radionuclides in the field of hematology.