The unique thermotolerance of hyperthermophiles, suitable as valuable sources of thermostable enzymes, is a result of various minor changes which led to the restriction of protein flexibility and modifications of nucleic acids and membrane lipids. Furthermore, all hyperthermophiles produce a number of heat shock proteins and thermoprotectants. Thermal resistance of these microorganisms is also enhanced by rapid resynthesis of thermolabile compounds and by elimination of such intermediates from cell metabolism.