[en] Fertilizer use efficiency is taken to be the percentage of fertilizer nutrient applied to a crop and subsequently taken up by that crop. Radioactive and stable isotopes (e.g. ³²P and ¹⁵N) provide the only means of obtaining a direct quantitative measure of the influence of various factors of fertilizer use efficiency by a crop. The direct isotope method and the indirect difference method for determining fertilizer nutrient uptake are compared. It is concluded that - while the indirect difference method may sometimes serve to make quantitative comparisons of different fertilizer sources or placement methods, it will rarely give a reliable quantitative measure of fertilizer use efficiency. For that, it is essential to use the direct isotope method. The coordinated research programmes of the Joint FAO/IAEA Division have included experiments on a wide range of annual and perennial crops. Amongst them were experiments on different methods and times of N application to maize; on the efficiency of different sources and times of N application to wheat; evaluations of efficiency of natural rock phosphates for rye grass and rice; and root activity studies on tree crops