After nearly three decades with little change in the treatment for B-cell non-Hodgkin’s lymphoma, the addition of immunotherapy has had a profound effect on the treatment of this group of diseases. A more subtle addition to the armentarium has been the radiolabeled monoclonal antibodies, 90yttrium ibritumomab tiuxetan and 131iodine tositumomab. Unfortunately these drugs have been underutilized. This is, in part, because of the need for coordination between specialties, concern about long-term effects, possible limitations on the tolerance of subsequent therapies and, in part, because of reimbursement factors. In this review, the studies in relapsed and refractory disease are discussed and the very promising results reported from phase II studies using radioimmunotherapy as first-line. Potential mechanisms of resistance to monoclonal antibodies are postulated based on alterations in cell signaling pathways that have been observed in lymphoma cell lines resistant to rituximab. It is anticipated that as mechanisms of resistance are better understood for both unlabeled and labeled monoclonal antibodies, biomarkers will not only predict their efficacy but also lead to the development of therapies to overcome resistance.