The issue of why only particular isotopes of uranium and plutonium are suitable for use in nuclear weapons is analyzed with the aid of graphs and semiquantitative discussions of parameters such as excitation energies, fission barriers, reaction cross-sections, and the role of processes such as [alpha]-decay and spontaneous fission. The goal is to provide readers with a ready reference for dealing with questions regarding which isotopes are of concern in discussions of weapons proliferation. (Contains 4 figures and 3 notes.)