Whether they are exposed to extremes of heat, cold, or buried deep beneath the Earth"s surface, microorganisms have an uncanny ability to survive under these conditions. This ability to survive has fascinated scientists for nearly a century, but the recent development of metagenomics and "omics tools has allowed us to make huge leaps in understanding the remarkable complexity and versatility of extremophile communities. Here, in the context of the recently developed metagenomic tools, we discuss recent research on the community composition, adaptive strategies and biological functions of extremophiles.