Targeting the CD20 antigen on B lymphocytes with the monoclonal antibody (MoAb) rituximab has greatly improved the outcome of patients with B-cell malignancies. Despite the success of rituximab, resistance occurs in about half of the patients, resulting in non-response to treatment or early relapse with the original disease. A better understanding of the mechanism of rituximab resistance has led to the development of novel, improved anti-CD20 antibodies. This review describes the development of CD20-targeted therapy from its historical background towards the next generation of anti-CD20 MoAbs and explains new strategies to overcome resistance. Semin Hematol 47:199-210. (C) 2010 Elsevier Inc. All rights reserved.