Understanding the sorption behavior of perfluorinated ionomer membranes and its effects on the membrane's transport properties remains an important issue in the development of ion-conducting materials. In this chapter, the current state of understanding of water-transport mechanisms in Nafion membranes are explained by summarizing the relevant studies in the literature as well as including some of our experimental results. A large discrepancy in the reported diffusion coefficient exists due to a number of techniques measuring the mobility of water molecules at different time- and lengthscales. In addition, morphological changes in the membrane during water uptake provide insight into the additional mechanisms of water transport and incorporation in the membrane. By analyzing the time constants from different experiments it is possible to draw key conclusions that shed light onto the multiple swelling and transport processes in membranes.