Alternate fusion fuels, i.e., fuels based on cycles other than d-t, are advocated because of apparent safety and environmental advantages, such as low activation of reactor materials and the relaxation of the requirement for tritium breeding that one needs for a d-t fusion reactor. Nevertheless, the lower fusion reaction rates and the higher required operating temperatures have suggested that the reactor performance would be inferior to that of a d-t reactor. This question of reactor performance relative to fuel cycle is examined here in the restricted context d-t versus d-d (with variations) In tokamaks, reversed-field pinches and tandem mirrors, although results relative to other concepts and cycles are reviewed. Each reactor concept is assessed relative to the relevant physics, engineering, cost and safety issues. There are distinct physics and technical leverages for each of the concepts, but many common features as well. For example, all three concepts require no blanket tritium breeding and have a much lower tritium inventory than their d-t counterparts, as well as, longer blanket lifetime, greater blanket efficiency, higher neutron energy multiplication and less activation. The physics constraints are not necessarily greater and cost per net power output between d-t and d-d reactors can be comparable.