[en] The International Nuclear Fuel Cycle Evaluation Study Report in 1980 identified over 150 research reactors in more than 35 countries, operating at powers between 10 kW and 250 MW using highly enriched uranium. Because of the associated proliferation risks, RERTR (Reduced Enrichment Research and Test Reactors) programs were established in a number of countries. Their aim was to investigate the feasibility and to develop technical means by which as many as possible of these reactors might be converted to use fuel of lower enrichment, preferably of less than 20% ²³⁵U, which is internationally regarded as an adequate isotopic barrier to weapons usability. A simplified, general review of some RERTR activities, including the technical principles of successful conversion and some associated fuel development programs is attempted in this paper. To illustrate conversion feasibility studies, a selection of results from enrichment reduction calculations for the AAEC's 10 MW Dido-type heavy-water-moderated reactor HIFAR and 100 kW UTR-type reactor Moata is presented. (author)