Non-myeloablative allogeneic stem cell transplantation (also known as mini-transplantation or reduced-intensity conditioning transplantation) is a major advance in the field of haematopoietic transplantation. This approach uses non-cytotoxic or reduced-intensity cytotoxic therapy to prepare patients for allografting of haematopoietic stem cells and lymphocytes. It has the potential to deliver the potent anti-tumour immunotherapy and bone marrow replacement capacity of allogeneic stem cell transplantation to patients with reduced treatment-related morbidity and mortality. It may also enable allogeneic transplantation in patients who would be considered ineligible for conventional transplants because of co-morbidity or advanced age. However, this approach may necessitate more careful monitoring of post-transplant chimerism and malignant disease-status than is usual with conventional allografting. There is also controversy regarding the best preparative regimen and graft-versus-host disease prophylaxis to use. This text should provide both practitioners and academic clinicians with a better understanding of underlying concepts, current methodologies and results obtained by the pioneering centres in this rapidly evolving field.