Devolatilization forms the major step in any thermochemical conversion process involving biomass materials. This is primarily due to the fact that biomass materials, on a weight basis, comprise about 80% volatile fractions and 20% solid carbonaceous residue. During the devolatilization or pyrolysis process for biomass materials, heat is applied to biomass particles through a direct or an indirect mechanism under inert or oxygen lean conditions. This process usually takes place in the temperature range of 300 —700°C and results in the liberation of volatile hydrocarbons from hemicellulose, cellulose, and lignin. These volatiles are subjected to thermal cracking or combustion depending on the reactor configuration. The pyrolysis process of biomass materials involves several reactions, which makes it a complex reaction phenomenon. Hence, in order to understand the pyrolysis mechanism of biomass particles as a whole, it is best to independently understand the pyrolysis mechanism of individual fractions present in the biomass sample, and then combine them to get an understanding of how a biomass sample as a whole undergoes the transformation. In this chapter, the discussion on pyrolysis of biomass components is presented along with several thermograms to give a general idea of thermal degradation of these components in terms of temperature range, kinetic behavior, and catalytic conversion.