The roles of protein, cytoplasmic protectants, and membrane lipid to stabilize bacteria at elevated temperatures by thermophiles are discussed. Acidophilic and alkaliphilic bacteria have special metabolic responses to withstand extreme environments by maintaining membrane charge and pH homeostasis. Unique physiological activities of irradiation resistance, halophilic, and psychrophilic bacteria are discussed. Information about the stress response of bacteria to osmotic shock, heat shock, and starvation is reviewed.