Treatment options for patients with indolent non-Hodgkin's lymphoma historically involved radiation or chemotherapy. Although initial response rates are excellent, treatment is increasingly less effective with each successive relapse. The advent of immunotherapy heralds a new era for the treatment of these patients. Radioimmunotherapy adds the benefits of cytotoxic radiation to immunotherapy and represents a significant addition to the treatment armamentarium. Various antigens for lymphoma have been targeted, of which anti-CD20 antibodies are the furthest in development. Ibritumomab tiuxetan (Zevalin; IDEC Pharmaceuticals, San Diego, CA), a (90)yttrium-labeled agent, and (131)iodine-labeled tositumomab (Bexxar; Corixa, Seattle, WA) are approved by the US Food and Drug Administration. Both agents have shown utility in therapy for relapsed and refractory low-grade and transformed lymphomas. This review highlights features of radioimmunotherapy that are relevant to non-Hodgkin's lymphoma, focusing on the two anti-CD20 antibodies.