Blood tumors have been the first cancers to find a treatment and a cure. It all started with the identification of chemicals capable of selectively killing cancer cells. Combined chemotherapies were proposed and, in some cases, proved very effective (e.g. Acute Lymphoblastic Leukemia in children). Over the years, chemotherapies significantly improved both in terms of efficacy and in the reduction of side effects. The introduction of Immunotherapy using Monoclonal Antibodies (MAB) and Bone Marrow Transplantation /Stem Cell Therapies have now become the mainstay of treatment not only for blood cancers but also for other cancers. These treatments are discussed in this chapter, where the issue of cell-to-cell compatibility in bone marrow/ stem cell therapies is also discussed in some detail.