This chapter provides a comprehensive overview of multi-layer composite (MLC) membranes for gas separation applications. Membrane technology has emerged as a viable and energy-efficient alternative to conventional gas separation methods such as cryogenic distillation and adsorption, due to its lower operational costs, modularity, and scalability. The growing interest in this technology has driven significant advancements in membrane materials and configurations, including the development of MLC membranes. This chapter delves into the fundamental principles of membrane technology, classifying different types of composite membranes and outlining their synthesis methods. Key gas transport mechanisms and theoretical models are explored to provide an overview into the functioning of these membranes. Practical applications in industries such as natural gas purification, biogas upgradation, post combustion carbon capture, helium enrichment, air separation and hydrogen purification are highlighted, demonstrating the versatility of MLC membranes. Additionally, the merits, including enhanced selectivity and permeability, durability and limitations, such as potential scaling challenges and long-term stability, are critically analyzed from the perspective of large-scale gas separation processes.