Bone marrow aspiration and biopsy are excellent techniques for evaluating bone marrow, but this evaluation is limited to a small part of the total blood-forming organ. With the introduction of radionuclide bone marrow imaging, a simple technique became available that overcomes marrow sampling errors by giving a total body view of functioning marrow. Furthermore, the procedure is noninvasive and provides an atraumatic method for evaluating a number of clinical problems including a discrepancy between bone marrow histology and clinical status (possible marrow sampling error), the determination of amount of active marrow after radiation and chemotherapy when further therapy is being considered, detection of sites of extramedullary hematopoiesis, location of the optimal sites for bone marrow biopsy, the diagnosis and staging of diffuse hematologic disorders, detection of metastases, the diagnosis of bone marrow infarcts in hemolytic anemias, and detecting avascular necrosis of the femoral heads. There are two major classes of bone marrow agents: (1) those that are incorporated into the erythroid precursors such as radioiron and (2) colloids that are taken up by the reticuloendothelial system (RES). Indium-111 chloride was originally considered to be an erythropoietic agent but appears to share some properties of RES labels. The best label to use is dependent on the disease being evaluated.