Polymers have been recognized to exhibit selective permeation rates to gases for more than a century. The commercial reality of this characteristic, however, occurred in the late 1970s. There has been significant commercial activity in this area which has also brought about a rapid increase in the study of polymeric structural variations to optimize the combination of gas separation and permeability (permselectivity). It has been recently noted that upper bound limits exist for the separation of common gas pairs. These limitations will be discussed along with the structural features necessary to achieve the best combination of high permeability combined with high selectivity. One of the speciality polymers receiving significant attention in the past decade is poly(trimethylsilylpropyne) (PTMSP) primarily due to a permeability to common gases an order of magnitude higher than silicone rubber. Structural variations, solvent variations, non-solvent swelling effects and flux decline of PTMSP are discussed. Flux decline, which has been reported in detail in the literature, is believed to be due to two factors. Contamination can significantly decrease permeability which comprises the reason behind many literature citations. Another factor involves a slow collapse of the structure (as cast) which can depend on the casting solvent. Non-solvent swelling promotes an instantaneous increase in permeability which slowly decays back to original values. High glass transition temperature engineering polymers (e.g. polyimides, polyarylates, polycarbonates) yield permselectivity characteristics of interest for gas separation. Structural features and variations of these polymers to achieve high permeability combined with selectivity (e.g. upper bound properties) will be discussed. Surface modification techniques comprise another route to achieve high selectivity for permeable polymers. These methods (e.g. fluorination, plasma modification, u.v. exposure) offer an additional route towards meeting the requirements for separation of gases with polymeric membranes.