A wide range of extremophilic microbes flourish in extreme environments where the conditions are harsh. Such conditions are considered fatal for other forms of terrestrial life. The extreme conditions are classified concerning the survival of microbes in habitats such as Antarctica to geothermal volcanic zones, marine to terrestrial, highly acidic zones to alkaline areas, regions exposed to elevated levels of radiation, and areas under high hydrostatic pressure. The extremophiles are of significance because they are considered a source for the discovery of useful bioactive compounds. The enzymes produced by them are of importance in various industries such as leather, paper, textile, food, and pharmaceuticals. These microbes are of importance for bioremediation purposes and are also known for their plant growth-promoting activities. This review describes microbial diversity in extreme conditions, its adaptation, and its applications.