Enzymes synthesized by thermophiles and hyperthermophiles are known as thermozymes. These enzymes are typically thermostable, or resistant to irreversible inactivation at high temperatures, and thermophilic, i.e. optimally active at elevated temperatures between 60 and 125 degrees C. Enzyme thermostability encompasses thermodynamic stability and kinetic stability. Thermodynamic stability is defined by the enzyme's free energy of stabilization (deltaG(stab)) and by its melting temperature (Tm). An enzyme's kinetic stability is often expressed as its halflife (t1/2) at defined temperature. DeltaG(stab) of thermophilic proteins is 5-20 kcal/mol higher than that of mesophilic proteins. The thermostability mechanisms for thermozymes are varied and depend on the enzyme; nevertheless, some common features can be identified as contributing to stability. These features include more interactions (i.e. hydrogen bonds, electrostatic interactions, hydrophobic interactions, disulfide bonds, metal binding) than in less stable enzymes and superior conformational structure (i.e. more rigid, higher packing efficiency, reduced entropy of unfolding, conformational strain release and stability of alpha-helix). Understanding of the stabilizing features will greatly facilitate reengineering of some of the mesozymes to more stable thermozymes.