A fusion-fission power system is considered in which fissile-producing hybrid (or fusion-fission) reactors supply the makeup fuel needs for fissile-consuming, or converter, reactors. As a result of the diverse nature of the hybrid reactor, a number of general performance parameters are proposed that will permit comparison of various hybrid reactor configurations. In addition, the relationships developed are used to show that scientific feasibility or “breakeven” for the hybrid reactor is near at hand in the fusion program and that attractive commercial reactors can be based on near-term plasma physics.