Pyrolysis is a thermal depolymerization and fragmentation process carried out at moderate temperatures in the absence of oxygen and with a wide range of reaction residence times. It can convert a wide range of solid materials, ranging from woody biomass and algae to waste plastics, into complex mixtures of low molecular weight organic molecules representative of the polymeric parent material. This chapter examines the chemical reaction kinetics of fast pyrolysis as well as fast pyrolysis process technologies, describing the transition from hydroprocessing to catalytic upgrading. The chapter describes pyrolysis co-products and their uses in the market and reviews the value of both technoeconomic assessment (TEA) and life-cycle assessment (LCA). Finally, the chapter looks at the commercialization and demonstration of biomass fast pyrolysis and looks ahead to future research trends in this area.