Computational fluid dynamics (CFD) approaches have proven valuable for examining the influence of filler shape, size and distribution on the behavior of MMMs and for assessing theoretical model assumptions. However, CFD models themselves require assumptions regarding the properties of the individual phases (polymer and filler) and the interfacial regions between them. Furthermore, the fundamental premise of the CFD approaches is to treat the fluids under consideration as continuous media, which is not necessarily true in the nanoporous membranes. Nevertheless, their primary value lies in the ability to perform parametric studies that quickly explore membrane behavior over a wide range of system parameters. In contrast, molecular simulations, though more computationally demanding, offer insights into system behavior at the molecular level without needing explicit assumptions about component properties. This makes molecular simulations especially useful for a detailed understanding of structure-property relations on the molecular level, including the role of the enhanced porosity in PIMs and the nature of the interfacial regions in MMMs.