All organisms need water to remain metabolically active. The availability of water in an environment is not simply a function of how much water is present, but the degree to which it is adsorbed to the insoluble components of the environment or chemically associated with solutes in that environment. For this reason, the concept of water activity, a measure of the availability of water to participate in chemical reactions, was devised. In general, lower water activity corresponds to lower microbial activity. Food scientists increasingly are seeking ways to exploit water activity in foods, in combination with other factors, to satisfy consumer preferences and the need for safe products with extended shelf life.