Polyimides (PIs) are an important, well-established, and commercialized class of polymers due to their extraordinary physical and chemical properties. They have been extensively applied as membrane fabrication materials for gas separation, especially in natural gas upgrading and acidic CO2 gas removal from industrial off-gases. However, two major unsolved challenges still remain for PI-based membranes: overcoming the trade-off relationship between the gas permeability and selectivity, and maintaining the long-term operational performance through controlling thermal and pressure conditioning, physical and chemical ageing, plasticization, swelling, permeation hysteresis, and resistance against impurities or presence of trace contaminants. This review aims to explore practical procedures to give the best insights into synthesis of efficient PI-based gas separation membranes as well as introducing advanced modification methods that have been applied for available PIs in view of obtaining a superior performance. A comprehensive “structure-to-property” relationship is elaborated by molecular design and engineering of PI monomers, i.e., the assembly of sub-objects: diamine and dianhydride monomers. This approach covers all issues from atom, functional group, segment (micro-structure or molecular design) to branch, chain and network assembly of the PIs. Detailed discussions include substitution positions, halogenated groups, bridging functional groups, bulky groups (linear and branched and subdivided into silyl and germyl, fluorine, methyl, iptycene and Tröger’s Base groups). Moreover, criteria for designing high quality hyperbranched polyimides (HB-PI), co-polyimides (co-PIs) including polyamide-imides, polyether-imide, triptycene based co-PIs, multi-block co-PIs, and hyper-branched co-PIs are presented. Cross-linked PIs are also discussed by classifying them according to the methods of reaction: thermal, UV, and chemical cross-linking (abbreviated by TCL, UVCL, and CCL, r
