A significant increase in our understanding of the dosimetry and therapeutic potential of various types of radioactive decay has augmented the possibility of utilizing specific radiolabeled carriers in tumor therapy. At least three types of radioactive decay lend themselves to this approach. Most fastidious in their distribution requirements are the Auger electron emitters whose radiotoxicity can only be realized with their intranuclear localization. Alpha emitters irradiate regions of several cell diameters. Beta emitters are less stringent as far as proximity to the total tumor mass, but doses to normal tissues can be high. The advantages and the limitations of each type of decay are presented.