Synthetic polymer membranes are enabling components in key technologies at the water–energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water–energy nexus require ion selectivity, or separation of specific ionic species from other similar species. In this talk, I will present strategies for materials design and process development to selectively extract target ions from a mixture containing other ions of similar size and charge. To design synthetic membranes and coatings with excellent selectivities for specific cations, we leverage concepts from naturally-occurring ion pores and channels which can separate ionic mixtures with selectivities greater than 1000 for similar ions (e.g. Na⁺ and K⁺). In one system, we achieve strong permeability selectivities for Cu²⁺ over other divalent ions through the use of layer-by-layer deposition of polymeric thin films containing iminodiacetic acid functional groups. The preferential binding of Cu²⁺ to these functional groups results in a strong enhancement of the permeability of Cu²⁺ relative to other divalent ions. In another example, we fabricate membranes and coatings with sulfonate groups using the conductive polymer formulation PEDOT:PSS and show that the permeability selectivity for Cu²⁺ over Na⁺ depends on the membrane crosslink density and water content. Next, we demonstrate how these materials and coatings can be incorporated into continuous-flow, electric-field driven separation processes to achieve targeted ion removal. By incorporating ion-selective films into a capacitive deionization process, we can achieve ion-selectivities as high as 14:1 on a mass basis. Using an analytical model for desalination of a mixed salt stream, we show that selective permeation of a target ion enables preferential removal of a specific ion. These studies demonstrate new concepts for the development of ion-selective membranes, and future work will leverage these findings to develop a more efficient process for copper recovery and for the recovery and purification of rare earth elements.