This chapter explores the mechanisms of mass transport in carbon-based membranes, focusing on how pore structure influences the movement of gas and liquid. Key transport models discussed include constrained diffusion, Knudsen diffusion, and viscous flow. Both experimental and theoretical approaches, such as solution-diffusion modeling, are examined to illustrate the effects of concentration, pressure, and temperature on transport behavior. Structural and compositional modifications, including zeolite integration and silicon doping, are shown to enhance permeability and selectivity. Overall, the chapter highlights the critical importance of precise pore control and consistent fabrication for achieving optimal membrane performance.