The potential clinical impact of radioimmunotherapy (RAIT) is being defined through laboratory investigation and phase I and II trials. For radiosensitive tumors such as Hodgkin’s disease, RAIT is more effective than currently available Phase I chemotherapy. The observation that RAIT can deliver 5 to 20 Gy to solid tumors insures its future use in combined therapy with external beam and/or chemotherapy. As a systemic therapy RAIT should have future application for treating metastatic, subclinical disease. It is clear, however, that the present generation of radioimmunoconjugates does not deliver sufficient radiation to justify use as single agent therapy for solid tumors. New immunoconjugates may substantially increase the deliverable dose. It is equally important to use these new agents in protocols that exploit the radiobiological response mechanisms of tumors and normal tissue, devising multifraction, multicycle regimens to produce not only differential dose deposition, but also differential biological response.