The levels of induced radioactivity in fusion devices can be controlled by appropriate selection of elements in a structural alloy and, in principle, by the selection of specific isotopes of a particular element. Three general rules are developed by which long-term induced radioactivity can be minimized. These rules are then applied to two specific alloy systems-stainless steels and the molybdenum alloy, TZM. A particular steel, Tenelon, containing neither nickel nor molybdenum, is especially attractive. It is found that the principles of both elemental substitution and isotopic tailoring can reduce the long-term radioactivity levels by orders of magnitude compared to normal Type 316 stainless steel. A comparison of long-term activity levels in such systems as the liquid-metal fast breeder reactor, fusion with standard structural alloys, and fusion with steel alloys designed for low activity quantitatively shows the potential advantage of fusion in this area. The influence of iso topic tailoring on gas production rates is also discussed. The calculations on radioactivity indicate that with proper attention to the choice of materials and isotopes, long-term radioactivity in fusion devices can be made so low as to either eliminate concern over long-term storage or allow recycling within a few human generations.