The introduction of trees that symbiotically fix atmospheric nitrogen is widely acknowledged as one of the most efficient means to sustain the productivity of agrosystems through the improvement of the soil nitrogen balance, especially in the tropics. Nevertheless, very few research programs specifically focus on this topic. Quite varied results have been obtained on fixed-nitrogen enrichment of the soil and use of this nitrogen to fertilize annual intercrops. The N2-fixing potential of tree species is often low or inhibited by environmental constraints such as drought, salinity, and excess mineral nitrogen or plant diseases. The symbiotic bacteria/host tree association could possibly be improved through plant biotechnology and genetic engineering research. Symbiotic nitrogen fixation could also be optimized by modifying current agricultural practices. Nitrogen-fixing trees can be intercropped or not with other annual crops in various conditions, e.g. hedges, bush fallows and long-term rotations. Such management programs should be better integrated in rural environments through the promotion of multipurpose species producing wood and high-protein livestock fodder as well as edible fruits or seeds.