Extremophilic microorganisms on Earth are groups of organisms that thrive in harsh environments and serve as important models for understanding evolutionary adaptation and genetic diversification of life. Many thermophiles have been isolated from composts and hot springs. Thermophiles have also been isolated from desert sand and sediments of deep-sea hydrothermal vents. Psychrophiles are found in polar regions, permafrost, and glaciers. Deep-sea environments are home to the psychrophilic piezophiles. Halophiles have been found in salt lakes, rock salts, and deep-sea high-salinity anaerobic basins. Acidophiles are found in acidic environments such as acidic hot springs and mine drainage, whereas haloalkaliphilic bacteria are isolated from hypersaline alkaline lakes. Bacteria of the genera Chroococcidiopsis and Deinococcus, which are resistant to desiccation, ultraviolet rays, and ionizing radiation, have been isolated from desert environments. Extremophilic microorganisms play a significant role in a wide range of research fields, including the study of the evolution of life, the possibility of extraterrestrial life, and applications in biotechnology.