Surgery is the most common treatment for solid tumors. If removed at an early stage of tumor development, the prognosis can be excellent. Unfortunately, in too many instances the tumor has already spread within the local environment. Although local progression of the tumor may still be controlled by surgery, the long-term survival prospects for the patient is in jeopardy, and with increasing evidence of more distal spread, such as lymph node involvement, the prognosis may be very poor. This may be improved if suitable methods are available to detect and eradicate metastatic sites. Using external scintigraphy, radiolabeled antibodies have been able to detect occult disease, thereby expediting possible intervention that may benefit patients. 1 7 However, radioantibody imaging, as well as conventional imaging modalities, are limited in their ability to detect tumors of less than 1 cm, and thus micrometastatic deposits of tumor will likely escape early detection. Radiolabeled antibodies have also been used in radioimmunoguided surgery to explore regions that otherwise would not have been examined. 8 , 9 Such exploration is limited to the immediate surgical field, and thus distal sites of tumor involvement may still go undetected. In addition, the sensitivity for detecting microscopic tumor foci is not known. In situations where there is the absence of any detectable tumor, patients may benefit from local irradiation of the region where the tumor was removed, but this type of treatment does not directly attack tumor cells that may have escaped from this local area. In other situations where there is a high suspicion that the cancer has already metastasized outside the local area, adjuvant chemotherapy may be employed. In this chapter, we will review preclinical data demonstrating that radiolabeled antibodies can effectively localize and kill micrometastatic nodules of colon cancer. In addition, radioimmunotherapy (RAIT) has been shown to be superior to conventional chemotherapy in treating micrometastatic disease in this preclinical model, thereby supporting a potential role for this procedure as an adjuvant cancer treatment.