This book provides a comprehensive and detailed source of information on the genetic and regulatory aspects of biological nitrogen fixation in free-living (non-symbiotic) prokaryotes. Biological nitrogen fixation is represented in a diverse range of microorganisms, among which Klebsiella pneumoniae serves as a paradigm for the genetic analysis of diazotrophy, which is the ability to grow with N2 as sole nitrogen source. The volume uses two major complementary approaches to the subject matter. The initial chapters use an organismic-based approach by concentrating on the well-characterized diazotrophic proteobacteria, cyanobacteria, Gram-positive clostridia, and Archea. The later chapters use a comparative process-based approach and serve as overviews dealing with different regulatory aspects, electron transport to nitrogenase, and molybdenum metabolism, across the range of organisms. Whenever appropriate, historical aspects and agricultural and ecological impacts have been taken into consideration. Each chapter contains an extensive list of references. This book is the self-contained second volume of a comprehensive seven-volume series. No other available work provides the up-to-date and in-depth coverage of this series and this volume. This book is intended to serve as an indispensable reference work for all scientists working in this and closely related fields, to assist students to enter this challenging area of research, and to provide science administrators easy access to vital relevant information.