Many elements have radioisotopes of radiotherapeutic potential. Based on considerations of type of emission, energy, half-life, production capability, and cost, a significant number have been evaluated for application to radioimmunotherapy. A listing of various of these radionuclides is provided in Table 1, adapted from Fritzberg and Wessels (1). As can be easily seen, the majority of the radionuclides that have been of interest are metals and a great deal of work has been carried out to stably attach them to antibody proteins in ways that do not interfere with the tumor targeting of the antibody. This chapter will describe the various approaches used for the attachment of metals to antibodies, evaluations in vitro and in vivo, and a perspective on current status of metallic radionuclides in radiotherapy. As space and time limitations preclude an exhaustive review of all studies of therapeutic radionuclides, the chapter will focus on those that are representative and have had significant developmental effort applied to them. Several reviews are available that describe properties and production of therapeutic radionuclides, and they are recommended for more details (2-4).