Alemtuzumab is a humanized monoclonal antibody against CD52, a small glycosylphosphatidylinositol-anchored glycoprotein that is highly expressed on normal T- and B-lymphocytes, and on a large proportion of malignant lymphoid cells, but not on hematopoietic progenitor cells. Over the past several years, a number of clinical trials have demonstrated the clinical activity of alemtuzumab in treating patients with chronic lymphocytic leukemia, T-cell malignancies such as T-prolymphocytic leukemia and cutaneous T-cell lymphoma, as well as in the prevention and therapy of graft-versus-host disease in the setting of allogeneic stem cell transplantation. Its application in a number of autoimmune disorders is currently under investigation. The most significant side effect of alemtuzumab is predisposition to infections related to the associated profound lymphopenia. Despite this, and with appropriate and more effective antibiotic prophylaxis, it is likely that we will witness an expansion of the role of alemtuzumab in the future.