The past decade has witnessed major revisions in our perception of the manner in which water affects the physical, chemical, and microbiological attributes of all manner of food-related systems. The growing realization that, during processing, most such systems are brought to, and maintained in, a state of thermodynamic instability is focussing attention on the dynamics of the various components in such systems. Older, equilibrium-based concepts, such as "water activity," equilibrium moisture sorption, and "bound" water are being discarded in favour of more appropriate descriptions, in terms of diffusion, nucleation, crystallization and relaxation rates, glass/rubber transitions, and steady states. It is being realized that food processing, materials science, and polymer technology have much in common, with water being the universal plasticizer of naturally occurring organic materials which form the basis of food products.