This chapter provides a comprehensive overview of gas transport in multilayer composite (MLC) membranes. It explored the transport mechanisms in the dense layer such as solution-diffusion, molecular sieving, and facilitated transport and their pivotal roles in enhancing separation performance. The discussion then turns to porous‐support phenomena, including Poiseuille flow, Knudsen diffusion, capillary condensation, and surface diffusion. This chapter also thoroughly explains the role of the gutter layer in enhancing MLC membrane gas separation performance. Several predictive models including Maxwell, Bruggeman, Lewis–Nielsen, Pal, and Cussler are critically evaluated with respect to their theoretical foundations, practical applicability, and limitations. The chapter also addresses challenges in non‑ideal mixed‑matrix membranes such as defects, uneven filler distribution, and the impact of concentration polarization. Lastly, the insights are discussed with recommendations for future research, emphasizing the potential of modeling tools to advance next-generation MLC membranes for industrial gas separation applications.