A synthesis of knowledge on the fixation of nitrogen by systems involving plants, especially legumes, and micro-organisms. Such associations utilize nitrogen from the atmosphere to make amino acids and proteins, and are of primary importance in providing a large part of the nitrogen compounds needed to sustain life. Originally published in 1976, this volume underlines, and in some respects quantifies, their importance in both natural and agricultural habitats. The text is divided into parts dealing with genetic aspects, legume inoculants, field assessment, the effect of the environment and with fixation in non-leguminous symbioses. The final chapter is devoted to more controversial areas.