Worldwide, aerobic suspended growth process has been widely used for the biological treatment of wastewater, the most common being the activated sludge process (ASP). The conventional ASP has several advantages like relatively low capital cost, high treatment efficiency, good process, and operational control. The limitations of the conventional ASP include requirement of large land area, high operating costs, and generation of large sludge volume resulting in higher disposal costs. To overcome these limitations, several modifications in the design of conventional ASP have been made. One of the widely used modification of the conventional ASP is the sequencing batch reactor (SBR). The SBR is an activated sludge system operating in “batch” mode. This chapter aims to cover the fundamentals of the ASP including the microbial ecology, growth kinetics, oxygen and nutrient requirements, and the classic reactor configuration. The chapter discusses the design and operating criteria for the conventional ASP in detail. It discusses the various operational problems associated with the conventional ASP along with the remedies. The chapter also provides an overview of various modified processes of the conventional ASP like extended aeration, tapered aeration, step aeration, and contact stabilisation. Next, the fundamentals of design of SBR process are discussed in detail. Finally, to connect the fundamentals with practical design, a design example is provided for SBR.