Reduced-intensity stem cell transplantation (RIST) has opened a new era for hematopoietic stem cell transplantation (HSCT). It was developed based on the knowledge that graft-versus-tumor (GVT) effect is the main anti-tumor effect in allogeneic HSCT. Because RIST is associated with less morbidity and mortality, it can be applied to many patients who could not undergo conventional HSCT. Experiences in the last decade clarified many issues related to RIST. For example, graft-versus-host disease (GVHD) in RIST may differ in character compared to conventional HSCT. Also, it is now known that intensity of conditioning is important in disease control, and the optimal regimens may be different for each disease or for each disease status. There are still many unsolved questions, and large prospective randomized trials are necessary to resolve these.