Water is frequently overlooked as a functional component of foods even though it can constitute anything between 95% of some (tomatoes) and 4% of others (milk powder). The formula H₂O greatly underestimates its complexity, especially in the liquid state where short-lived structured clusters of 200–300 molecules occur. The interactions between water molecules account for its anomalous properties, e.g. very high boiling point. The interaction between water molecules and the molecules of food components such as sugars and polysaccharides, protein, and fats explain the properties of many foods, including gels and meat products. Our understanding of the interaction of water with food materials is dominated by the concepts of water activity (a_W) and sorption isotherms. The measurement of water's concentration and activity in food products presents special problems to the food analyst. The chapter concludes with a list of specialist books and review articles for further study.