The importance of bone marrow transplantation for patients who do not have a matched sibling donor cannot be overestimated. This subject has always been in the public domain, accentuated by dramatic appeals, from time to time, to search for matched volunteer donors in the public at large. Unfortunately, the availability of such donors is limited, due to the remarkable genetic diversity of humans. Thus, although registries of such volunteers now include more than seven million individuals, we still face the problem of finding a matched donor for about 30% of patients in need. To address this burning issue, extensive clinical and basic research is performed in leading institutes around the world. This book presents updated accounts of the different aspects of this research. The scope of the book is very wide, including strategies to overcome graft vs. host (GVH) disease and graft rejection, cell therapy to prevent leukemia relapse, and a range of modalities to improve immune reconstitution after transplantation. In addition, new approaches to induce immune tolerance towards organ transplants by means of hematopoietic stem cell transplantation is extensively reviewed.