Polymeric membranes for gas separation exhibit an intrinsic trade-off between gas permeability and selectivity, i.e., polymers with higher permeability tend to have lower selectivity. To overcome this conundrum, polymers have been doped with ionic liquids (ILs) to enhance their gas permeability and selectivity, since ILs have very low vapor pressure and high CO2 solubility and permeability. The effect of IL doping on polymer morphology (such as glass transition temperature, melting temperature and polymer crystallinity) and gas transport properties is reviewed, and quantitative models are presented. In general, IL doping depresses melting temperature and crystallinity, which improves gas permeability. Such an effect is exemplified in semi-crystalline cellulose acetate (CA) and cellulose triacetate (CTA), which have been used to prepare commercial membranes for CO2/CH4 separation. IL doping can decrease the crystallinity in CA and CTA and increase CO2/CH4 solubility selectivity, resulting in enhanced CO2/CH4 and CO2/N2 separation properties. With appropriate ILs, doping provides an effective route to overcome the intrinsic trade-off of permeability and selectivity in polymers to achieve superior separation properties.