Water-mediated ion conduction enables high conductivity in hydrated polymer membranes commonly used in electrochemical devices. Understanding the coupling of the absorbed water with the polymer matrix and the dynamics of water inside the polymer network across the full range of length scales in the membrane is important for unraveling the structureproperty relationships in these materials. By considering the water behavior in ion-containing polymers, next-generation fuel cell membranes are being designed that exceed the conductivity of the state-of-the-art materials and have optimized conductivity and permeability that may be useful in other types of devices such as redox flow batteries. Waterpolymer associations can be exploited to tune the transport and mechanical property tradeoffs in these polymers. Measurements of water motion provide important criteria for assessing the factors that control the performance of these types of materials. This review article discusses current understanding of water behavior in ion-containing polymers and the relationship between water motion and ion and molecular transport. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011