Agricultural productivity is largely determined by the availability of nitrogen, added as fertilizer or introduced by microbial nitrogen fixation. A doubling of the World's population early in the next century will require a doubling of the effective agricultural nitrogen input. Economic and environmental constraints will probably preclude multiplying the input of industrial nitrogen fertilizer appropriately; so biological fixation, which is an exclusively microbiological process, must be exploited more effectively. Short term prospects include the expanded use of existing systems, such as exotic legumes, grasses and woody symbioses. In the medium term, plant breeding and genetic manipulation of the appropriate bacteria should yield more effective symbioses. In the long term, new nitrogen-fixing systems might be developed: somatic hybridization of plants might yield new symbiotic systems; manipulation of genetic information for nitrogen fixation into the plant genome might yield plants able to fix nitrogen independently of bacteria.