The turn of the millennium from the twentieth to the twenty-first century provides an occasion to review our understanding of a biological process, biological nitrogen fixation, that is of prime importance for the continued survival of mankind. This process has provided a basis for maintaining soil fertility since the beginning of organised agriculture, yet its very existence was confirmed only just over a century ago. In the intervening years, an enormous intellectual effort has dispersed much of the mystery surrounding biological nitrogen fixation. Biological fixation is widely exploited in agriculture, as are nitrogen fertilisers prepared for the last hundred years under extreme conditions of temperature and pressure. However, despite all our efforts, the fundamental nature of the reactions involved at the heart of the biological process remain unknown. This book aims to describe what we have learned in the last one hundred years or so about biological nitrogen fixation, about what its chemistry appears to be, and how it is applied in agriculture. This ambitious objective has not been attempted recently. It is aimed at students and those who wish to enter these very challenging areas of research, and who need to learn the state of the art at the turn of the millennium. The authors are all acknowledged world experts in their fields. They have prepared concise, well referenced and authoritative accounts of their subjects. This book provides a unique summary of the current state of knowledge that will be indispensable to all students and researchers, actual and potential, interested in biological nitrogen fixation.