Inorganic radioisotopes form the basis of FDA approved molecular imaging agents and radiotherapy agents. For molecular imaging, radiopharmaceuticals that emit g rays or positrons that penetrate through the body are employed. On the other hand, radioimmunotherapy (RIT) agents employ radioisotopes that emit a or b particles that possess limited range in tissue. Technetium-99m (^99mTc), a g emitter, is currently the most widely used radioisotope in the clinic for diagnostic imaging due to its favorable physical properties (g energy: 142 keV; half-life: 6.02 hr) and convenience (obtained from a ⁹⁹Mo/^99mTc generator). Recent targeted imaging agents, AcuTect (^99mTc apcitide) and NeoTect (^99mTc depreotide), are based on cyclic peptide targeting vectors and peptide chelators that offer amine and amide nitrogen and thiolate sulfur donor atoms to stabilize a Tc^V oxo moiety. The development, characterization and chemistry under physiological conditions of these agents will be discussed. Rhenium-188, the third row congener of technetium, as well as radiolanthanides are potentially useful for radiotherapy applications because these isotopes offer a range of b particle energies and half-lives that may be matched to the size and disposition of tumors in the body and to the residence times of targeting vectors. Parameters that impact the stability of ¹⁸⁸Re chelates will be elucidated.