Catalytic pyrolysis is a thermochemical conversion method in which biomass is decomposed at elevated temperature in using a heterogeneous catalyst to selectively produce bio-oil, char, and other value-added chemicals in inert atmosphere. The composition and yield of products highly depends upon the constituents of biomass, feed size, heating rate, temperature, and residence time. In addition, the choice of catalyst has also a critical role in enhancing the yield and composition of the resulting bio-oil and co-products. This chapter presents an in-depth study of biomass catalyst pyrolysis, emphasizing the influence of key operational parameters and the obtained products. It also discusses the roles of different catalysts in both catalytic pyrolysis and co-pyrolysis technologies.