A variety of microbes inhabit extreme environments. Extreme is a relative term, which is viewed compared to what is normal for human beings. Extreme envi-ronments include high temperature, pH, pressure, salt concentration, and low temperature, pH, nutrient concentration and water availability, and also condi-tions having high levels of radiation, harmful heavy metals and toxic compound(organic solvents). Cul-ture dependent and culture-independent (molecular) methods have been employed for understanding the diversity of microbes in these environments. Extensive global research efforts have revealed the novel diver-sity of extremophilic microbes. These organisms have evolved several structural and chemical adaptations, which allow them to survive and grow in extreme en-vironments. The enzymes of these microbes, which function in extreme environments (extremozymes), have several biotechnological applications. Antibiotics, compatible solutes and other compounds obtainable from these microbes are also finding a variety of uses.