Polymer composites are integral to industries such as aerospace, automotive, construction, and marine due to their exceptional mechanical properties, lightness, and adaptability. However, ensuring their long-term durability and reliability under complex service conditions—including mechanical loading, thermal cycling, and environmental exposure—remains a significant challenge. This chapter presents a novel multi-scale, multi-physics framework that integrates experimental investigations with advanced numerical modeling to elucidate the degradation mechanisms of polymer composites. Key findings include the identification of critical damage modes—such as matrix cracking, fiber-matrix debonding, delamination, and environmental degradation—and their interactions under realistic thermomechanical loading conditions. The chapter also presents state-of-the-art experimental techniques and modeling strategies that provide new insights into the durability and longevity of these materials. By addressing existing gaps in the understanding of polymer composite degradation, this work provides practical solutions to improve the structural integrity and lifetime of these materials in demanding industrial applications.