Lignin, a complex and heterogeneous biopolymer found in plant cell walls, is required for structural stability and microbial resistance. As one of the most abundant organic polymers, lignin’s characteristics and stability are of great interest in materials research, bioengineering, and environmental science. This chapter provides an in-depth review of lignin’s morphological, chemical, thermal, and biological characteristics. We discuss the intrinsic factors, such as bond types and functional groups, and extrinsic factors, including environmental conditions, chemical reagents, and biological agents, that influence lignin stability. Thermal stability, with decomposition temperatures ranging from 200 °C to 600 °C, is analyzed through thermogravimetric analysis (TGA) profiles. Chemical stability in acidic, alkaline, and oxidative conditions, as well as biological stability against enzymatic and microbial degradation, is all investigated. Strategies to enhance lignin stability through chemical modifications and environmental optimization are examined. This chapter aims to provide an in-depth knowledge of the elements influencing lignin stability and techniques for improving it, thereby realizing lignin’s potential in sustainable and innovative industrial uses.