Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems,the value of leguminous crops lies in their ability to fix atmospheric N2, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N2 fixing legumes provide the basis for developing sustainable farming systems thatincorporate integrated nutrient management. By exploiting the stable nitrogen isotope 15N, it has been possible toreliably measure rates of N2 fixation in a wide range of agro-ecological field situations involving many leguminousspecies. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and theirRhizobium symbionts which can be used to enhance N2 fixation. Practical agronomic and microbiological meansto maximise N inputs by legumes have also been identified.