Although the control of water activity (aw) is perhaps the earliest means of preserving food, a number of recent trends and developments in food science are likely to have a significant impact on the way it is used in the future. The development of predictive microbial models and quantitative risk assessment is leading to a paradigm shift in the way that food safety risks are managed internationally. This will lead to the use of glass transition temperature (Tg) as a well-defined control measure within a risk management framework. Another recent development in food preservation is the use of nonthermal processes for pasteurizing and sterilizing foods; the safe application of these new food processes will require that the effect of Tg is well understood. Finally, developments in whole cell genomics give the potential to examine which genes or gene products are switched on in response to microbial stresses. As with other preservative factors, it is hoped that these techniques will allow a more detailed mechanistic understanding of the effects of reduced aw on microbial cells to be developed.