The use of monoclonal antibodies (MoAbs) as targeting vectors for use in radioimmunoscintigraphy is becoming an established clinical procedure. 1 6 The extension of the technique from γ-emitters to particulate-emitting radionuclides for use in radioimmunotherapy (RAIT) is the focus of much current research, the chemical aspects of which will be the topic of this chapter. That research is of a diverse chemical nature as the twenty or so potentially useful particulate radiation emitters are scattered throughout the periodic table. There are over 1200 isotopes of the elements, and so more isotopes than not are radioactive. However, many are precluded from consideration as potential RAIT agents due to having an unsuitable half-life; as any isotope with a half-life outside the 1-h to 50-d range would certainly be inappropriate; and even the breadth of this range is probably overly generous. Also, many isotopes are not useful because of high intensity γ-radiation accompanying their particulate emissions. Still others may be suitable from a radiophysical standpoint, but cannot be prepared conveniently or in sufficiently high specific activity for use with MoAbs. A number of articles discussing the suitability and relative merits of various radioisotopes for RAIT, from the viewpoint of their radiophysical properties, have been written. 7 , 15