Bone marrow transplantation is now being performed in children having a variety of hematologic malignancies and solid tumors. Marrow donors for patients with hematologic malignancies are usually allogeneic, including HLA-identical siblings, one-antigen mismatched family members, unrelated matched donors, or in some situations, two- or three-antigen mismatched family member donors. Umbilical cord blood is being explored as a source of hematopoietic reconstitution for some allogeneic transplants. Recipients with solid tumors most often receive autologous marrow or PBSC grafts. Posttransplant complications continue to include acute and chronic GVHD, infections, prolonged immunodeficiency, and recurrent malignancy. Because children are now surviving longer after transplantation, a variety of delayed effects are becoming apparent. These include, but may not be limited to, neuroendocrine dysfunction, neuropsychological effects, and ocular and pulmonary dysfunction. Secondary malignancies are now also becoming apparent, particularly among patients surviving more than 10 years after transplantation. Despite these known problems, marrow transplantation remains the treatment of choice for patients who relapse from conventional chemotherapy and for patients with CML in chronic phase and AML in first remission. Research continues to develop methods to decrease posttransplant complications and, hence, increase the probability of long-term disease-free survival.