The gas separation membrane field has seen exceptional growth since its inception, with extensive research being performed to find novel, high performance polymer membrane materials for application in industrial separations. In this review, we examine recent advances of polymer design strategies to overcome the permeability/selectivity tradeoff relationship and the diminished performance caused by physical aging. We explore fundamental structure–property relationships within current state-of-the-art polymer materials to discern promising routes for continued membrane materials research and development, highlighting promising macromolecular architectures and design strategies providing tailorable polymer free volume for high performance gas separation membranes. Finally, we provide our perspective on encouraging design routes moving forward.