Book excerpt: Metagenomics is the study of microbial communities sampled directly from their natural environment, without prior culturing. DNA sequencing (or alternatively, an assay of functional properties) is applied to the culture-independent analyses of complex and diverse ("meta") populations of microbes. The rapid growth of metagenomics entails an expanded focus on microbes in a wide range of basic and applied research fields, ranging from microbiology to agriculture, biomedicine to bioenergy, and from veterinary to clinical medicine. This book is an insightful and inclusive study on metagenomics and its applications.