The nitrogen (N) cycle is one of the most important nutrient cycles on the planet, and many of its steps are performed by microbial organisms. During the cycling process, greenhouse gases are formed, including nitrous oxide and methane. In addition, the use of nitrogen fertilizers increases freshwater nitrate levels, causing pollution and human health problems. A greater knowledge of the microbial communities involved in nitrogen transformations is necessary to understand and counteract nitrogen pollution. This book - written by renowned researchers who are specialized in the most relevant and emerging topics in the field - provides comprehensive information on the new theoretical, methodological, and applied aspects of metagenomics and other 'omics' approaches used to study the microbial N cycle. The book provides a thorough account of the contributions of metagenomics to microbial N cycle background theory. It also reviews state-of-the-art investigative methods and explores new applications in water treatment, agricultural practices, climate change, among others. The book is recommended for microbiologists, environmental scientists, and anyone interested in microbial communities, metagenomics, metatranscriptomics, and metaproteomics of the microbial N cycle.