Adsorption plays a pivotal role in advancing the efficiency of multilayer composite (MLC) membranes for gas separation applications. This chapter delves into the fundamentals of adsorption, highlighting its mechanisms such as physisorption and chemisorption and its distinction from absorption. The unique ability of adsorption to selectively capture specific molecules based on surface interactions is discussed in comparison to other separation mechanisms, such as molecular sieving and solution-diffusion. Key theoretical models, including Langmuir, Freundlich, and Brunauer–Emmett–Teller (BET) isotherms, as well as adsorption kinetics, are explored to provide a comprehensive understanding of adsorption dynamics. The chapter further investigates the critical influence of membrane surface properties, including morphology, porosity, functionalization, and surface chemistry, on adsorption performance. Various factors such as environmental conditions, material properties, and adsorbate characteristics are analyzed for their impact on adsorption efficiency in MLC membranes. Experimental methods, from gravimetric analysis to advanced spectroscopic techniques, are reviewed to demonstrate their role in characterizing and optimizing adsorption in membrane systems. The discussion also addresses current challenges, such as the trade-offs between adsorption and diffusion mechanisms, and explores future opportunities for enhanced adsorbent materials and membrane designs. This chapter discussed the importance of adsorption in the development of high-performance MLC membranes and offers insights into the potential advancements in gas separation technologies.