Bringing together the diverse disciplines that study nitrogen fixation, this book describes advances made in various fields: chemists are now studying FeMoco, the active site of nitrogenase in non-protein surroundings, and the crystal structure of the enzyme has been refined to 1.6 angstroms. Advances in the complex regulation of nitrogen metabolism and nitrogen fixation gene expression in the free-living, associative, endophytic, symbiotic and photosynthetic diazotrophs are detailed, as well as factors involved in the nodulation process and nodule metabolism in legumes. In recent years molecular techniques have expanded phylogenetic studies and genome sequencing. Extensive studies on biological nitrogen fixation in sustainable agriculture, particularly in the tropics, environmental stress on plants and microbes, rhizobial strain selection, methods of soil reclamation and newly discovered plant bacterial associations are described. Finally, the possible avenues of nitrogen fixation research in the coming century, including the expression of nitrogen fixation genes and the establishment of nitrogenase function in plant organelles, the prospects of developing nitrogen fixation in rice and the development of resistant transgenic legumes are explored. All these developments were discussed at the 21st International Congress on Nitrogen Fixation held at Foz do Iguassu, Parana State, Brazil, and are covered in these proceedings.