Membranes and membrane technology are key to the nation’s water and energy security. Polymer-based membranes already play a significant role in fields such as water purification, fuel cells and batteries, where they control the selective transport of ions and/or water. The exact nature of ion and water transport is often ambiguous and depends on the complex interplay between polymer structure and dynamics that facilitate transport. To this end, we are developing and applying advanced measurement methods to quantify structure/property/transport in polymer-based membranes to address the grand challenge of the energy-water nexus.