Thermionic cell development is progressing at such a rapid rate that some of the data required for incorporation of thermionic converters into reactor systems is already becoming available. Although such information is not yet sufficient for detailed design and performance evaluation of nuclear-thermionic systems, it is adequate for preliminary analysis. As more experimental information is obtained, these preliminary analyses will have to be reviewed and the concepts reevaluated. However, they have already shown the interesting potential of—and the severe problems to be overcome in—applying nuclear-thermionic systems in space, marine, and central-station power plants. In this article, the possible concepts for utilizing thermionic cells with nuclear reactors are reviewed and the feasibility of their applications is discussed.