Enzyme inactivation during heat processing is reviewed with regard to fundamental aspects (structure, thermodynamics and kinetics), mathematical models and the relationship between enzyme activity and food quality. Enzyme stability is related to enzyme structure and to factors in the microenvironment. Kinetics of inactivation are categorized with respect to reaction order and two models are briefly discussed which describe the variation of inactivation rates with temperature. The determination of accurate kinetic parameters is emphasized where the objective is to develop mathematical models of enzyme inactivation in real foods. The value of modelling inactivation is assessed. Enzymes having effects on sensory quality are reviewed with particular emphasis on those enzymes which influence flavour, through lipid degradation, and those which affect texture by catalysing the breakdown of starch, pectin or protein.