The development of radionuclide therapies during the past few decades provides a growing body of data on radiobiologic effects, including normal tissue toxicities and antitumor efficacy. Information on normal tissue toxicity from radionuclides is more limited than that from external beam radiation and appears to be more variable. Much of the increased variability is attributed to heterogeneous distribution, which complicates the potential for whole-organ toxicity, and the differences in dosimetry methodology. Although new tools are becoming available, quantitation of heterogeneous dose for radionuclides is usually less precise than dosimetry that is used in external beam radiation practice. The correlation between reported dose estimates and toxicity has improved during the past 2 decades, partly as the result of increased accuracy and standardization of dosimetry techniques and to adjustment for biologic effects. This review provides an updated compendium of dose-response relationships and consideration of dosimetry as well as radiobiologic factors that influence the reported results. Data presented are mainly derived from studies involving deliver of radiation to adults with malignancies, with most experience from radionuclides that predominantly emit beta radiation.