Enzymes can lose activity through covalent and noncovalent structure alterations. In the former, protease attack and modification by small active molecules such as oxygen are important. Conformational stability can be measured by Tm, the midpoint temperature of the thermal denaturation curve, and turnover in vivo of a number of enzymes correlates with Tm. Measurement of Tm and delta Cp leads to evaluation of delta H, T delta S, and delta G for the unfolding process. The importance of d(delta G)/dT is emphasized since it can be used to evaluate the temperature of maximum stability. There is no simple relationship between amino acid sequence and delta Gmax, nor can the effect of mutation be accurately forecast. Reversibility of folding is an important factor in stability. Tm correlates with [G]1/2, the midpoint guanidine unfolding concentration, and is the most useful predictive quantity for enzyme stability.