This article is a broad overview of hematopoietic stem cell transplantation, covering topics such as indications for transplantation, types of transplantation procedures, HLA matching of donors and recipients, procurement of stem cells from donors, and outcomes of transplantation. The major complications of transplantation, such as graft-versus-host disease, and the mechanisms of tissue injury after transplantation are discussed, as are the nature of immunologic impairment and reconstitution after transplantation. Finally future directions in stem cell transplantation are discussed.