The fluidized bed biofilm reactor (FBBR) represents a recent innovation in biofilm processes. Immobilization of microorganisms on the small, fluidized particles of the medium results in a high reactor biomass holdup which enables the process to be operated at significantly higher liquid throughputs with the practical absence of biomass wash-out. The process intensification (i.e., a reduction in process size while maintaining performance) achieved in FBBRs makes this innovative technology particularly attractive in biological wastewater treatment, commercial biomass conversion, and ethanol and biochemical production applications. In this chapter, the present understanding of biofilm phenomena involved in the operation of FBBRs is reviewed. Special emphasis is placed on the microbial and kinetic aspects of FBBRs and practical design considerations and current applications are described.