The humanized anti-CD52 monoclonal antibody alemtuzumab belongs to the family of Campath-1 antibodies, which were initially developed for their ability to prevent graft-versus-host disease (GVHD) and graft rejection in stem cell transplantation. Alemtuzumab is indicated for the treatment of chronic lymphocytic leukaemia (CLL) and has demonstrated considerable activity in relapsed/refractory disease and in previously untreated disease. It has been shown to induce minimal residual disease-negative responses as a single agent or as part of consolidation therapy in a meaningful proportion of patients with CLL and has shown promising activity in patients with high-risk cytogenetic markers. Alemtuzumab may also have significant activity in T-cell malignancies, such as mycosis fungoides and T-cell prolymphocytic leukaemia. Recent studies also have evaluated alemtuzumab as part of a conditioning regimen to prevent GVHD in stem cell transplantation. This article reviews our current understanding of alemtuzumab and discusses its emerging role in the treatment of CLL and other haematological malignancies.