Hematopoietic Stem Cell Transplantation (HSCT) is an established treatment for a variety of malignant and nonmalignant conditions. In autologous HSCT, the stem cells are collected from the patient then infused back after high dose chemotherapy. On the other hand, in allogeneic HSCT, the stem cells are donated from another individual (who may be related or unrelated and unmatched or matched). In this book, the authors present new research in the study of hematopoietic stem cells including pulmonary complications following HSCT; psychological health and adjustment of families of children who undergo HSCT; the immunogenetics of unrelated HSCT for thalassemia; HSCT for inflammatory bowel diseases; and extramedullary hematopoiesis leading to the production of a novel antigen-presenting cell type in the murine spleen.