As presently conceived at Oak Ridge National Laboratory and described in this issue, the single-fluid Molten-Salt Breeder Reactor, operating on the 232Th-233 U fuel cycle and based on a reference design, has a breeding ratio of ∼1.06, specific fissile inventory of 1.5 kg/MW(e), a fuel doubling time of ∼20 years, and fuel cycle costs of ∼0.7 mill/kWh(e). Start-up may be accomplished with either enriched uranium or plutonium, with little effect on fuel cost; the breeding ratio, averaged over reactor life, is reduced 0.01 to 0.02 relative to the equilibrium cycle. Operated as a converter, with limited chemical processing, the reactor may have a conversion ratio in the range 0.8 to 0.9 with fuel cycle costs of 0.7 to 0.9 mill/kWh(e).