It has been 45 years since the first indication that radioimmunotherapy (RIT) might be a feasible approach to therapy of cancer. Since then, especially during the past decade, many antibodies directed against tumor-associated antigens have been produced, successfully labeled with radionuclides, tested in xeno-graft models, and evaluated in Phase I clinical trials. Gamma-emitting radionuclides have been used to assess biodistribution and tumor targeting, and beta emitters have been used with therapeutic intent, but with limited success in solid tumors. To place RIT of solid tumors in context, this chapter will first briefly describe the historical aspects of radiolabeled antibodies, use of diagnostic radiolabeled monoclonal antibodies (radioimmunoscintigraphy), and the role of nonimmune, internally administered beta emission radiotherapy in the management of cancer patients. We shall then consider the choice of radionuclides and antibodies for RIT and preclinical RIT studies. Finally we will focus on clinical trials of RIT of those solid tumors not discussed in other chapters.