Lignin, as the second most abundant component in lignocellulosic biomass, was previously considered a waste by-product; however, recent studies have indicated that it can be used for various applications. Due to the complexity of lignin structure and variations in lignin’s extraction methods, it is difficult to characterize and investigate lignin’s thermal and chemical behavior and properties. To address this, extensive studies have utilized a variety of analytical techniques such as DTG, DSC, TGA, FTIR, Raman, GC-MS, and NMR along with TGA-FTIR-GCMS and TGA-DT-GCMS to understand lignin thermal and chemical properties. In this chapter, initially, an introduction to lignin and its application is presented. Then, the chemistry of lignin and different classifications are discussed based on the sources and extraction processes. After this, different thermal characterization methods, in tandem with some structural characterization and their combination, are studied to better understand lignin’s chemistry, reaction pathways, and thermochemical properties and behavior. In conclusion, these thermal characterizations would give valuable knowledge regarding the change in the chemistry of lignin and thermal degradation of lignin over a range of temperatures.