Lignin is a highly abundant natural biopolymer derived from plants, and its utilization in various applications is crucial for advancing sustainable bioeconomic practices. However, due to limited hydroxyl group content, the overall reactivity of lignin is relatively low, hindering its further exploitation. Demethylation can serve as a key process to overcome this issue. Demethylation can eliminate one or two methoxyl groups from the ortho positions of lignin and introduce phenolic hydroxyl groups. This significantly boosts the chemical reactivity and biodegradability of lignin. To date, many chemical and biological methods have been developed to demethylate lignin. In this chapter, the significance and mechanism of lignin demethylation will be elucidated, and various approaches commonly used for lignin demethylation will be summarized. These include those involving acids, nucleophilic reagents, alkali metals, photoredox catalysis, microorganisms, and enzymes. Moreover, the pros and cons of different methods will be compared and discussed. This chapter provides a foundation for understanding lignin demethylation and developing more efficient strategies.