Metagenomics is one of the fastest advancing fields in biology. By permitting access to the genomes of entire communities of bacteria, viruses, and fungi - otherwise inaccessible - metagenomics is extending our comprehension of the diversity, ecology, evolution, and functioning of the microbial world, as well as contributing to the emergence of new applications in many different areas. The continual and dynamical development of faster sequencing techniques, together with the advancement of methods to cope with the exponentially increasing amount of data generated, are expanding our capacity for the analysis of microbial communities from an unlimited variety of habitats and environments. The synergism with the new emerging omics approaches is showing the path to functional metagenomics and to adopting integrative, wider viewpoints, like systems biology. This book covers the most innovative and recent advances in theoretical, methodological, and applied areas of metagenomics. The topics