Radioimmunotherapy is a form of targeted radionuclide therapy that uses a monoclonal antibody to deliver localized radiation. It is most appropriate for treatment of multiple tumor sites that cannot be readily excised surgically or irradiated using external beam radiation or brachytherapy. At present, 2 products, Bexxar ((131)I-tositumomab and unlabeled tositumomab, GlaxoSmithKline, Triangle Park, NC) and Zevalin ((90)Y-ibritumomab tiuxetan and unlabeled rituximab, Spectrum Pharmaceuticals, Irvine, CA and Cell Therapeutics, Seattle, WA) are approved for treatment of non-Hodgkin's lymphoma in certain clinical situations in the United States and Canada. Zevalin is available also in Europe, and there are plans to make both agents more widely available. The therapeutic dose to be used depends upon a number of patient-specific variables. Both regimen achieve a complete response or partial response in approximately 3 of 4 patients, with a duration of remission lasting many years in some cases. This article reviews the basis for dose selection, the nuclear medicine procedures involved, the results obtained to date, and issues related to patient and staff safety.