Allogeneic hematopoietic stem cell transplantation (HSCT) currently provides the only chance of curative therapy for many patients with hematological malignancies. Owing to the excess morbidity and mortality observed in less robust patients, this approach has traditionally been limited to younger patients without significant medical co-morbidities. Given the age profiles of patients with candidate hematological malignancies, a majority of patients are therefore ineligible for myeloablative therapy. Over the past few years the ability to achieve hematopoietic stem cell engraftment using immunosuppressive but non-myeloablative conditioning regimens has made it possible to consider a much broader group of patients for allogeneic HSCT. The discovery that the hematopoietic graft itself can contribute to the eradication of malignant cells through a graft-versus-malignancy (GVM) effect, independent of the conditioning regimen, has led to investigation of reduced intensity conditioning regimens for a variety of malignant diseases. Over the past few years there has been a dramatic increase in the number of patients undergoing reduced intensity conditioning for both malignant and nonmalignant conditions. This review will focus only on the use of such therapy for malignant disease.