Polymer membranes are important for a range of aqueous solution-based technologies that require controlled rates of ion transport. Advanced membranes will need to be engineered at the molecular level to control interactions among the polymer, sorbed water, and ions that come into contact with the membrane. Currently, little is known about the nature of these interactions, and broadband microwave dielectric relaxation spectroscopy (DRS) is a tool that could provide critical dielectric permittivity property information to facilitate modeling and understanding of the ion sorption and transport properties of hydrated polymer membranes. A handful of microwave DRS studies on hydrated polymers are reported in the literature, and these studies represent a critical first step toward using DRS to unlock important structure-property relationships. This review discusses microwave DRS and research focused on understanding the influence of polymer chemistry and water content on relative permittivity and ion transport properties of hydrated polymer membranes.