Mixed-matrix membranes (MMMs) with metal–organic frameworks (MOFs) as additives (fillers) exhibit enhanced gas permeabilities and possibly also selectivities when compared to the pure polymer. Polyimides (Matrimid®) and polysulfones are popular polymer matrices for MOF fillers. Presently investigated MOFs for MMMs include [Cu(SiF6)(4,4′-BIPY)2], [Cu3(BTC)2(H2O)3] (HKUST-1, Cu-BTC), [Cu(BDC)(DMF)], [Zn4O(BDC)3] (MOF-5), [Zn(2-methylimidazolate)2] (ZIF-8), [Zn(purinate)2] (ZIF-20), [Zn(2-carboxyaldehyde imidazolate)2] (ZIF-90), Mn(HCOO)2, [Al(BDC)(μ-OH)] (MIL-53(Al)), [Al(NH2-BDC)(μ-OH)] (NH2-MIL-53(Al)) and [Cr3O(BDC)3(F,OH)(H2O)2] (MIL-101) (4,4′-BIPY = 4,4′-bipyridine, BTC = benzene-1,3,5-tricarboxylate, BDC = benzene-1,4-dicarboxylate, terephthalate). MOF particle adhesion to polyimide and polysulfone organic polymers does not represent a problem. MOF-polymer MMMs are investigated for the permeability of the single gases H2, N2, O2, CH4, CO2 and of the gas mixtures O2/N2, H2/CH4, CO2/CH4, H2/CO2, CH4/N2 and CO2/N2 (preferentially permeating gas named first). Permeability increases can be traced to the MOF porosity. Since the porosity of MOFs can be tuned very precisely, which is not possible with polymeric material, MMMs offer the opportunity of significantly increasing the selectivity compared to the pure polymeric matrix. Additionally in most of the cases the permeability is increased for MMM membranes compared to the pure polymer. Addition of MOFs to polymers in MMMs easily yields performances similar to the best polymer membranes and gives higher selectivities than those reported to date for any pure MOF membrane for the same gas separation. MOF-polymer MMMs allow for easier synthesis and handability compared to pure MOF membranes.