This chapter presents the activated sludge process (ASP) as the most widely applied biological wastewater treatment technology. It outlines the three main stages—primary, secondary, and tertiary treatment—before focusing on the ASP as a suspended growth system that couples aeration and microbial activity for organic matter removal. Core components such as aeration tanks, aeration systems, and secondary clarifiers are described, emphasizing their roles in pollutant degradation and biomass recycling. The chapter introduces modeling approaches to predict process performance and explores key operational parameters, including hydraulic retention time (HRT), solids retention time (SRT), sludge age, and mixed liquor suspended solids (MLSS). Practical design aspects, such as maintaining optimal F/M ratios and preventing sludge bulking, are also highlighted, linking microbial kinetics with engineering operation and compliance standards.