Methods currently available to the clinician for the detection and localization of cancer are not specific and may have reached their technical limits. Deficiencies of these methods are detailed, and an ideal system for cancer detection is presented. Monoclonal antibodies directed against tumor antigens and labelled with radionuclides approach the ideal as a method for cancer screening, detection, localization, monitoring, and, perhaps, therapy. The production and radiolabelling of monoclonal antibodies are reviewed, and recent technical developments for imaging are explained. Methods to manipulate the system to achieve specific goals are outlined, and examples are shown. Metastases of certain tumors are demonstrated. The method holds great potential for diagnosis, monitoring, and even for radiotherapy of target tumors. Clinical trials are now in progress.