Molten-salt reactors (MSRs), because of the fluid nature of the fuel, appear to provide an attractive approach to efficient fuel utilization in the thorium-233U cycle as well as a means for limiting the availability of plutonium and the general proliferation risks associated with nuclear power generation. High-enrichment 233U systems could, in principle, be operated with positive breeding gains to effectively eliminate plutonium as a nuclear fuel However, such systems would be proliferation sensitive. Concept modifications (short of denaturing the uranium fuel) can be conceived to enhance the proliferation resistance of high-enrichment MSRs, but it is doubtful that sufficient enhancement could be achieved to make the systems suitable for deployment other than at “secure” sites. Denaturing the uranium in an MSR introduces some plutonium into the fuel cycle and generally degrades its breeding performance. Nevertheless, a denatured MSR with full-scale on-site fuel reprocessing appears to be capable of break-even breeding. In addition, the plutonium (most of which is consumed in situ) would be of poor quality and would never be isolated from all other undesirable nuclides. Thus, such systems would provide for efficient utilization of uranium resources in a proliferation-resistant environment while limiting the amount of plutonium (and transplutonium actinides) that would have to be handled as waste. The development of commercial MSRs by early in the 21st century appears to be technologically feasible.