Metagenomic analysis has extraordinary potential to improve our understanding of microbial populations in their natural environment and identify novel genes of interest. The key feature of such analyses is that they are performed using metagenomic libraries constructed from total DNA isolated from a particular niche rather than a laboratory culture. Thus, metagenomic analyses potentially allow access to all the genetic resources present in an environment, regardless of whether or not they belong to microorganisms that can be cultured in the laboratory. Sequence-based metagenomic analyses rely on comparisons with databases of known genomic sequences whilst functional analyses rely on screening libraries on the basis of the phenotypes cloned DNA can confer to host bacteria. Therefore, functional analysis allows the identification of novel genes with functions that could not have been predicted from their DNA sequence. This book discusses metagenomics' methods, applications and perspectives.