Silk, a protein-based fiber made of polymers of fibroin and sericin, has long been considered as the best textile material. Apart from the textile industry, silk has found its applications in various other fields. In the biomedical field, it is used for tissue engineering, delivering therapeutic agents and in optics and sensing. To fully utilize the material properties of silk, it is essential to know about its thermal degradation. As elevated temperatures cause structural changes in silk proteins, it is important to understand their thermal degradation for effective processing at high temperatures. Chemically, silk is stable under normal conditions but when exposed to extreme pH or harsh environmental conditions, it becomes unstable. Silk’s diverse applications can be attributed to its tissue compatibility, biodegradability, relatively less processing compared to synthetic alternatives, and outstanding mechanical properties including high tensile strength, elasticity, and flexibility. The mechanical power of silk arises from its chemical constitution, hierarchical arrangement, and hydrophobic regions that stimulate the shaping of antiparallel β-sheet crystals. The chapter discusses structural features and mechanical aspects of silk fibroin and sericin, focusing on fibroin’s β-sheet crystalline structure and sericin’s globular nature. It also addresses the thermal and chemical stability of silk proteins, emphasizing their resilience under various environmental conditions and highlighting their broad applications in textiles and biomedicine.