Monoclonal antibodies have been a part of biomedical research for nearly a decade, yet a comparable period of time may be required before radiolabeled monoclonal antibodies come into standard practice in the diagnosis and therapy of human disease. Difficulties in identifying appropriate antigens, developing the optimal antibody species, attaching the most effective radiolabel and choosing the best method of administration all confound present investigations. The problems confronting therapeutic applications are, for the most part, an extension of those associated with diagnostic techniques. Still, there is much well-deserved optimism, and progress in all areas of investigation will ultimately come together to resolve most if not all of these problems. It is inevitable that monoclonal antibodies will have significant impact on the practice of nuclear medicine, and presented here is a review of the basic knowledge as well as an update on the current status of radiolabeled monoclonal antibodies in medicine.