This chapter describes reactors for thermochemical conversion of lignocellulosic biomass into fuels, energy, and chemicals. The chapter covers basic definitions and concepts involved in biofuels and thermochemical conversion of biomass, and it also includes more advanced topics such as the main reactor configurations currently in use for thermochemical technologies, important parameters for reactor design, discussion of how parameters affect reactor performance, and several examples and case studies. The focus is on fast pyrolysis and gasification systems. The topics discussed include energy and carbon efficiencies, convenience of operation and scale-up, and several other parameters related to reactor design. After reading this chapter, the reader will understand the main characteristics of reactors for thermochemical conversion of biomass, their strengths, and their weaknesses for specific applications.