Various fusion fuel cycles are reviewed in view of a commercial fusion reactor. Among exothermic fusion reactions, D-3He fusion appears attractive because it mitigate engineering problems of DT fusion attributed to 14MeV neutrons. The safety of the reactor is inherent and environmentally sound with D-3He fueled fusion. Nevertheless, the helium-3 resource is very rare and attempts to breed helium-3 seem unsatisfactory. Lunar helium-3 alone seems to provide the world a needed energy.