To optimize the productivity of ion exchange membranes used in electric field-driven ion separation processes, an understanding of the relationship between membrane structure-property metrics and a measure of ion separation is necessary. The membrane separation factor is a commonly used indicator of ion separation efficacy, and as outlined in this review, it can be related to the intrinsic sorption and diffusion selectivity properties of the membrane. Doing so connects the separation factor to key theories that describe ion transport, and this connection facilitates an analysis of the implications of these theories on electric field-driven ion separations. The process of electrodialysis and ion exchange membranes can be applied for both desalination and ion separation applications, and this review discusses relationships between commonly used metrics for electric field-driven transport (e.g., transport number) and properties commonly used in desalination contexts (e.g., sorption and diffusivity selectivity). These relationships provide context for commonly observed experimental trends. Additionally, some common assumptions (and their implications for describing membrane transport properties related to a multicomponent ED system) are discussed. This review also links fundamental membrane properties (such as sorption and diffusivity selectivity) to ion separation-critical properties (such as the ion exchange affinity). While the diffusivity selectivity may be more important at lower current density values, the sorption selectivity is expected to be important across a wider range of current density values. This review further highlights the interconnected manner by which ion exchange membrane properties and external process conditions couple to influence ion separation performance.