The processing and storage of foods result in changes due to physical processes and chemical reactions. The rates of these changes depend on many factors, but the mobility of the reactants and of other significant food components is often a major factor controlling these rates. Recently, it has been recognized that the glass transitions of food components are important in controlling mobility. This recognition led to the development of "state diagrams" in which glass transition temperatures (T-g) of major food constituents provide a boundary between regions of low mobility (glasses) and increasing mobility ("rubbers"). This approach has been particularly useful in predicting rates of shrinkage, recrystallization, textural changes and volatile (flavor) loss in carbohydrate-based foods. Work on chemical reactions (non-enzymic browning and lipid oxidation) has also demonstrated the profound effect of glass transitions on the rates of these reactions. The common unifying factor is the effect of glass transition on the diffusivities of important reactants in the materials which undergo the transition.