Controlling moisture content in food, by either eliminating water content or binding it so that food is stable to both microbial or chemical deterioration, has been practiced for more than 3,000 years. The FDA has included the concept of water activity in its good manufacturing practices (GMP). Water activity is defined as the vapor pressure of water in food divided by vapor pressure of pure water at the same temperature. Water activity controls chemical reaction rates and the order of chemical and kinetic reactions in foods, on local phase rates, and on the lipid reaction rate. It is impossible, with the present state of the art, to predict precise shelf life because of the variability of food systems. However, mathematical models can be constructed, based on data collected at high humidity and high temperature conditions, to predict shelf life under normal storage conditions.