Microorganisms that convert gaseous nitrogen (N2) to a form suitable for use by living organisms are pivotal for life on Earth. Another set of microbial reactions utilize the bio-available nitrogen creating N2 and completing the cycle. This crucial nutrient cycle has long been the subject of extensive research, and recent advances - in studying the biochemistry, bioinformatics, cell biology, and the physiology of bacterial nitrogen cycling processes, alongside the advent of the omics age - have had a massive impact, enabling us to fully appreciate the sheer diversity of approaches adapted by individual organisms. Research in this area is at a very exciting stage. This timely book provides comprehensive reviews of current nitrogen cycle research and gives a broader perspective on the state of our understanding of this key biogeochemical cycle. With contributions from expert authors from around the world, the topics covered include: the archaean N-cycle * redox complexes N-cycle * organization of respiratory chains in N-cycle processes * Mo-nitrogenase * nitrogen assimilation in bacteria * alternative routes to dinitrogen * nitrite and nitrous oxide reductases * assembly of respiratory proteins * nitric oxide metabolism * denitrification in legume-associated endosymbiotic bacteria * nitrous oxide production in the terrestrial environment * bacterial nitrogen cycling in humans. This book will serve as a valuable reference work for everyone working in this field and will also be of interest to researchers studying symbioses, environmental microbiology, plant metabolism, infection events, and other prokaryote-eukaryote interactions.