Water, as the main component of food and biological materials, plays a predominant role in determining their shape, structure, and physical and chemical properties. It also is a major control component in mass transfer, chemical reactions, and the activity of microorganisms. The food technologist involved in product development, process design, or production is confronted daily with the effect, desirable or undesirable, of water on the food material. In either case, a better understanding of the water relations with the food would certainly be of help. Most of the unit operations used in food processing have as a goal, in one way or another, either the removal of water to stabilize the material, as in drying and concentration, its transformation into a nonactive component in freezing, and its immobilization in gels, structured foods, and low and intermediate moisture foods. The main and essential way in which the immobilization of the water is measured is through the consideration of aw and its relationship to moisture content. Based on the thermodynamic concept of water chemical potential in solutions, aw has served and still is serving as an index of how successful we are at controlling water behavior in food systems. It is also the parameter that controls the driving force in water removal operations and is therefore essential for design purposes. Its effect on many of the other properties important to food manufacturing and preservation will be reviewed in the subsequent chapters in this volume. What all this indicates, however, is that we need to be able to deal with aw quantitatively if we want to effectively control our food processing.