This review provides a new prospective on the role of the state-of-the-art polymers of intrinsic microporosity (PIMs) in key energy-intensive membrane-based gas separations including O2/N2, H2/N2, H2/CH4, CO2/CH4, H2S/CH4, C2H4/C2H6, and C3H6/C3H8 applications. A general overview on the gas separation properties of novel PIM materials developed in the past 15 years is presented with updated performance maps on the latest pure-gas 2015 O2/N2, H2/N2, and H2/CH4 permeability/selectivity upper bounds. Specifically, functionalized ladder PIMs and polyimides of intrinsic microporosity (PIM-PIs) are discussed targeting at high-performance, plasticization-resistant membranes for demanding acid gas (CO2 and H2S) removal from CH4 in natural gas and olefin/paraffin separations. Experimental CO2/CH4 performance data of nearly 70 polymeric membrane materials available in the literature were gathered and plotted for the first time on the Robeson plot, from which a mixed-gas 2018 CO2/CH4 upper bound was proposed to provide guidance for future membrane materials development. A number of PIMs have demonstrated outstanding performances in O2/N2, H2/N2, and H2/CH4 separations, and several functionalized PIMs have shown great promises in CO2/CH4 separation under realistic mixed-gas conditions. The potential of PIMs materials and their derivatives for H2S/CH4, C2H4/C2H6, and C3H6/C3H8 separations are underexplored and significant efforts are needed to develop stable and high-performance materials under mixed-gas conditions. Ultimately, fabricating PIMs materials into defect free, inexpensive thin-film composite or integrally-skinned asymmetric membranes is paramount to their successful large-scale commercialization.