We believe that hematopoietic stem cell transplantation (HSCT) will help subjects with Osteopetrosis generate functioning osteoclasts, and by so doing assist in the resolution of the abnormal bone architecture, and the anemia and bone marrow failure that is also characteristic of this disease. However, we have found in past studies that approximately 30% of Osteopetrosis patients do not engraft. Therefore, in this study, we plan to use a different combination of pre-transplant drugs to try to make transplants safer for this disease, as well as to provide a second infusion of stem cells in patients with matched related or unrelated donors. The purpose of this research is to find a safer and more effective means of performing stem cell transplantation in patients with Osteopetrosis, using chemotherapy and radiation designed to bring about engraftment and lessen transplant mortality.