Raman spectroscopy, as a rapid, high-precision, and nondestructive tool, can be used for analyzing the samples from gas to solid, from ex situ to in situ, from organic macromolecule to minerals. It has been demonstrated as a powerful tool for characterizing carbonaceous solid fuels and their thermal conversion products. This review provides a systematic overview of the application of Raman spectroscopy for investigating the entire thermochemical processing of coal, biomass, and wastes. After introducing the fundamentals of Raman spectroscopy, its application for characterizing the feedstock (raw coals, biomass, and wastes) is reviewed. Then, using the Raman spectroscopy for ex situ characterization of the products (char and ash) after reactions and in situ diagnostic during reactions are discussed. Besides, some potential advanced Raman spectroscopy techniques are further briefly introduced. Lastly, the challenges and prospects of using Raman spectroscopy to study thermochemical processes are discussed.