Acute complications such as veno-occlusive disease of the liver, acute and chronic graft-vs-host disease (GVHD), and infectious conditions remain major obstacles for the success of allogeneic hematopoietic stem cell transplantation (HSCT). Progress in allogeneic HSCT depends on several factors, including the adequate prevention and management of associated complications, advances in the conventional management of diseases currently treated with allogeneic HSCT, expansion of the donor pool, selective control of GVHD, development of more effective preparative regimens to eradicate the neoplastic cell population, characterization of a new generation of hematopoietic growth factors and cytokines, and development of newer techniques for ex vivo manipulation of stem cells. Hematopoietic growth factor-mobilized donor progenitor cells collected from peripheral blood have been shown to be associated with rapid hematopoietic engraftment without an increase in the incidence of acute GVHD compared with allogeneic bone marrow transplantation. Implementation of this approach will enhance donor acceptance, eliminate the risk of general anesthesia, decrease cost, and reduce the risk of infectious complications by reducing the duration of neutropenia. Nonmyeloablative allogeneic stem cell transplantation represents a novel treatment approach that may lead to reduced toxic effects and extended use of this treatment in older patients and in those with malignant and nonmalignant disorders. However, GVHD and disease recurrence remain a challenge. Promising results have been reported in patients with refractory hematologic malignancies and in metastatic renal cell cancer. Because late complications are commonly encountered in patients receiving allogeneic HSCT, lifelong observation is needed.