The possibility to calculate in a predictive way the relevant transport properties in glassy polymeric membranes is inspected in detail with particular attention to the methods recently developed based on solid thermodynamic foundations. The solubility of pure and mixed gases, diffusivity and permeability of single gases and vapors are examined both for more traditional polymers as well as for high free volume matrices of great interest as PIM1 and modified PIM matrices. The procedures considered clearly indicate the relevant physical properties of the polymer matrix and of the penetrants which are required, and that can be obtained experimentally through independent measurements. For gas and vapour solubility, the comparison with direct experimental data for mixed gases points out also the ability to account for the significant variations that solubility-selectivity experiences upon variations of pressure and/or feed composition. For gas and vapour permeability, the comparison with direct experimental data shows the possibility to account for the various different trends observed experimentally as penetrant pressure is increased, including the so-called plasticization behavior.