Nitrogen fixation is currently of great practical importance because the use of nitrogenous fertilizers has resulted in unacceptable levels of water pollution and the fact that these fertilizers are becoming steadily less economic both in cash terms and in utilization of fossil fuels for their manufacture. This book aims to present new findings in the field of nitrogen fixation and to reflect the influence on the subject of recent developments in biochemistry, molecular biology, genetic engineering and other biotechnologies. The book includes quantitative aspects of the subject, but the many practical difficulties for accurate field assessment of nitrogen fixation and its contribution to nitrogen cycles means that many assessments are either in conflict with each other or must be accepted as tentative. Although this book is concerned essentially with how nitrogen-fixing organisms function and why they are of practical importance, rather than with details of laboratory or field techniques, an appendix deals with the main methods of measurement of nitrogen fixation and some of the problems that must be faced.