The fixation of nitrogen--the conversion of atmospheric nitrogen to a form that plants can use--is fundamental to the productivity of the biosphere and therefore to the ability of the expanding human population to feed itself. Although the existence and importance of the process of biological nitrogen fixation has been recognized for more than a century, scientific advances over the past few decades have radically altered our understanding of its nature and mechanisms. This book provides an introductory-level survey of biological nitrogen fixation, covering the role of the process in the global nitrogen cycle as well as its biochemistry, physiology, genetics, ecology, general biology and prospects for its future exploitation. This new edition has been fully updated to include the most recent developments in the field, providing an up-to-date and accessible account of this key biological process.