Lignin is the most abundant aromatic biopolymer on Earth and serves as an essential structural component in vascular plants. It is produced on a large scale—over 50 million tons annually—as a by-product of the pulp and paper industry, although only a small fraction is commercially utilized. This chapter provides a comprehensive overview of lignin’s ecological functions, availability, chemical structure, physicochemical properties, and its degradation and valorization pathways. It highlights conventional industrial applications such as resins and dispersants, as well as advanced uses in bioplastics, nanomaterials, energy storage, and green chemistry. High-value lignin-derived products include carbon fibers, UV-protective films, smart materials, and biomedical hydrogels. Emerging technologies such as enzymatic processing, catalytic conversion, and additive manufacturing are discussed, along with digital tools that facilitate the design of lignin-based materials. Overall, lignin is positioned as a strategic renewable resource with growing relevance across diverse industries and scientific domains.