Membrane based separation is developing quickly due to low energy consumption, high efficiency, and easy operation. Metalâorganic frameworks (MOFs) are a new class of molecular-sieve materials composed of metal ions or metal-oxide clusters coordinated by organic linkers to form highly regular porous networks. Owing to their adjustability in both pore apertures and functionality, MOFs in the form of membranes have offered unprecedented opportunities for efficient gas separations. A number of MOF membranes have therefore been fabricated and shown superior performance in H2 purification, CO2 capture, and C3H6/C3H8 separation. Moreover, in comparison with other candidates like zeolites, graphene oxides (GOs), and carbon molecular sieves, the presence of coordinatively unsaturated metal sites, guest-induced gate-opening phenomenon and framework flexibility in some MOF materials further endows them with infinite possibilities for efficient separation of a series of gas mixtures with similar kinetic diameters (like CO/N2, C2H2/CO2, and C2H4/C2H6 separation), which is otherwise quite challenging. Unceasing pursuit of materials with superior selectivity, permeability, and long-term operation stability remains the permanent themes in the field of membrane science and technology. In the case of MOF membranes, pore aperture (selectivity), functionality (selectivity), grain boundary defects (selectivity), thickness (permeability), and binding strength (operation stability) represent the most critical factors influencing their performance. Therefore, microstructural engineering, which typically includes orientation manipulation, interfacial synthesis, and construction of mixed-phase membranes at a mesoscopic level, and architectural design, which typically contains aperture size adjustment, cage modification, and post-decoration at a microscopic level, have become indispensable for performance improvement of MOF membranes.