This chapter discusses the bioreactor as the core unit of biological wastewater treatment, providing controlled environments for microbial growth and pollutant transformation. It explains the classification of bioreactors into suspended growth and attached growth systems, along with operational modes such as batch, continuous, and fed-batch. Mass balance principles are introduced as essential tools for quantifying substrate utilization, biomass production, and sludge generation. The chapter further examines key reactor models including CSTRs and plug-flow reactors, emphasizing how design parameters like hydraulic retention time (HRT) and solids retention time (SRT) govern system performance. Applications in chemostats, activated sludge processes, and F/M ratio calculations are explored, linking microbial kinetics with practical engineering design.