Electrochemical technologies play a crucial role in industries such as water treatment, resource recovery, energy storage, and power generation. With the growing demand for desalination, hydrogen fuel, and storage of renewable energy, the significance of these technologies will only continue to increase. Central to each of these applications are ion-exchange membranes (IEMs), which are polymers that enable essential ion transport. Given their widespread use in electrochemical devices, advancements in IEM design can significantly enhance the performance of technologies across various industrial sectors. To improve IEMs, it is crucial to understand the current state of their development and design. This was accomplished by analyzing extensive data sets comprising 40 commercially available IEMs and >1000 membranes reported in the literature. By integrating this diverse data set with state-of-the-art thermodynamic and transport models, we produced the Donnan-Meares-Manning model for the upper bound of selective ion transport in IEMs. This model revealed that tortuosity and charge dilution scale equally and oppositely with the water content of membranes, leading to the observed trade-off between charge selectivity and counter-ion conductivity. Additionally, analysis of this model was able to pinpoint the most promising strategies for enhancing IEM performance: increasing charge contents and increasing diffusion selectivities. The most direct route to achieving superior ion transport properties was increasing the polymer charge content. Following this direction, we produced ultrahigh charge density (UHCD) IEMs. These unprecedented materials achieved combinations of fast and selective ion transport previously unreached by commercial and reported IEMs. Case studies on the most selective and most conductive UHCD IEMs refined this design strategy. Specifically, increases in charge contents beyond 8 mol / L[polymer] incurred strong electrostatic penalties which mitigate many of the expected improvements in ion transport properties. Similarly, across multiple morphological membrane structures, increases in water contents beyond 0.66 L[water] / L[membrane] led to a reduction in ion diffusivities due to the generation of non-conductive water domains. Both of these findings provide practical guidance for the continued study of UHCD IEMs and help understand the forces governing ion transport in these materials. The Donnan-Manning-Meares model also indicated that improved diffusion selectivities would improve membrane performance; however, it is not clear how to prepare membranes meeting this criterion. To develop a better fundamental understanding of how to control the diffusivity of target ions within membranes, we examined the energetics of ion diffusion for 15 ions in representative IEMs. We observed strong ion-specific effects, finding a 0.5 – 13 kJ/mol increase in the energy barrier for ion diffusion in IEMs containing various counter-ions. Supplemental experiments and molecular dynamics simulations helped to isolate the ion softness as a causal predictor of the energy barrier for crossing these membranes. According to Pearson’s Hard/Soft Acid/Base theory, the ion softness reflects electrostatic interactions through the malleability of ion hydration shells, suggesting that varying structure of polymeric fixed charge groups will be essential for regulating diffusional selectivity between similar ions. These results could be the first step towards achieving finely tunable control over ion diffusivities in IEMs.