Metagenomics has revolutionized microbiology by paving the way for a 
cultivation-independent assessment and exploitation of microbial communities 
present in complex ecosystems. Metagenomics comprising construction and 
screening of metagenomic DNA libraries has proven to be a powerful tool to 
isolate new enzymes and drugs of industrial importance. So far, the majority of 
the metagenomically exploited habitats comprised temperate environments, such as 
soil and marine environments. Recently, metagenomes of extreme environments have 
also been used as sources of novel biocatalysts. The employment of 
next-generation sequencing techniques for metagenomics resulted in the 
generation of large sequence data sets derived from various environments, such 
as soil, the human body, and ocean water. Analyses of these data sets opened a 
window into the enormous taxonomic and functional diversity of environmental 
microbial communities. To assess the functional dynamics of microbial 
communities, metatranscriptomics and metaproteomics have been developed. The 
combination of DNA-based, mRNA-based, and protein-based analyses of microbial 
communities present in different environments is a way to elucidate the 
compositions, functions, and interactions of microbial communities and to link 
these to environmental processes.